Answered 4/23/18:
I somewhat doubt he was just lucky, too. However, perhaps the attackers just underestimated him, and they thought he was dead… didn’t stop to make absolutely sure. So… maybe he actually WAS just lucky, as far as the STORY goes.
There’s a PLOT DEVICE reason for him to live, though, of course. He doesn’t seem to be able to tell our earl who the attackers were, but he is later needed (plot-wise) to explain to present company (the other Phantomhive servants) about the fact that there were twins. Our earl can’t get the (hardly any) words out, due to shock. Sebastian can’t give the full story because he wasn’t present before they were born or when they were little. Madam Red is already dead by this point in the story, and the Midfords are somewhat preoccupied elsewhere.
And, yes… my Mother3 “Theory of Everything” predicted (about 2 years ago) that he would turn out to be the character parallel to Leder. As I’ve mentioned before, Leder is the Tazmily Village bell ringer; he’s completely silent (until much later) and extremely tall. When he finally speaks, he explains that out of all the Tazmily villagers (new settlers on Nowhere Islands), he’s the only one who was allowed to keep his memories. All the rest were stripped of their memories of their lives before coming to Nowhere Islands on a huge white ship. The idea was to start fresh after an apocalyptic event elsewhere. [The Egg of Light (or Hummingbird Egg) was used to wipe/store their memories.] He was selected because of his grand height; when the time is right, the person who can fulfill the prophesy must seek out Leder to gain knowledge of their history. **He had taken an oath to remain silent until that moment.** So, Lucas, Boney, Kumatora/Violet, and Duster happen to find Leder trapped in the sewers of New Pork City; Leder has been chained up like some wild animal. I don’t think they are even able to free him right then, but they learn about the Dark Dragon who sleeps under the islands, the 7 needles that keep it incapacitated, the Magypsies who guard those needles, and the reinforcement of the idea that Lucas is the “chosen one”. (This is SO Harry Potter searching out the 7 guarded horcruxes to make the “Dark Lord” defeat-able that I’m laughing right now as I type this. I really need to finish my Mother3 masterposts and my triple-comparison posts between BB, the Mother series, and HP. Anyways….)
So, here’s how that all compares to BB. Tanaka is the only person to even survive the attack at Phantomhive Manor, besides the twins, and we know that real Ciel only lives about a month longer. Instead of acting mute and being extremely tall, Tanaka only says anything beyond “Hoh hoh hoh” when he has something important to say… and even though he’s actually a regular height, he’s often (particularly at the begining) depicted in super deformed or deformed/chibi form. Either way, he definitely stands out for not being talkative and for looking unusual. So, Phantomhive Manor and its surrounding estate become the parallel to Tazmily Village. Besides our earl (Lucas), Tanaka (Leder) is the only person to survive the attack at the manor and its aftermath about a month later. The other people who live on the surrounding lands (the earldom) just stay hush-hush about the twin situation, probably out of respect, once our earl returns to the manor. Same goes for others who knew there were twins, like Madam Red and the Midfords. Discussing it, particularly around our earl, would be cruel; discussing it around the Phantomhive servants would be wholly improper. Instead of going into some long-ago history about the Phantomhive family, Tanaka says he had **planned to take this truth to his grave**, and then he just (FOR NOW, anyway) explains the unexpected arrival of (mirror) twins into the household and how they were pretty much inseparable. We don’t see him chained up anywhere (like Leder is)… again “yet”… but he is being forced to serve his “Lord Ciel” as butler, and he doesn’t seem to pleased about it.
Only time (and future chapters) will tell whether Tanaka suffers further at the hands/orders of real Ciel (or someone else, like Lord Polaris/”Knife Dude”/butler to Lord Sirius)… or whether he ends up spilling the beans to our earl (and Sebastian) about some other historical/family knowledge. Even though the other people hearing this, right now, are other Phantomhive servants, I still think that Edward Midford will step back in as the parallel to Duster, and Gregory Violet will step back in as the parallel to Kumatora/Violet.
BONUS: Ch66 has such an interesting parallel to all this, since it’s about an Easter Egg hunt, in which Lizzie makes it a game for everyone to hunt for her painted egg. She falsely claims it’s something they did when they were younger, when the fact is she’s never done this before. It’s a set-up to test out our earl’s memories, and he fails. Afterwards, Lizzie is left wondering what could have possibly happened to “Ciel” during the month while he was away after the attack at the manor. Of course, everyone is determined to find the painted egg first, including her brother, Edward. Later, Edward talks our earl into helping him find Lizzie. Our earl later talks Edward into joining the Phantom Five, a five-piece band (in which he has to take on a new persona that is completely unlike himself), and it’s all in the name of finding and saving his little sister. Both of these “hunts” involve Lizzie. The first time they (actually Sebastian) **track Lizzie down, knock her out, and bring her back to the townhouse, she sneaks out and runs away again**. (Side notes: 1. We know that Lizzie is very picky about what shoes she wears around our earl. 2. Lizzie is the “genius with the sword”, not Edward. 3. “Edward Vision” is Edward’s ability to see only Lizzie in a room full of people. LOL)
In Mother3, there are two hunts for the Egg of Light (Hummingbird Egg), and both of them revolve around Duster. In Mother3, he grabs the egg the first time it’s found, but then he gets separated from the others in his team; it erases his memories to the extent that he suffers full amnesia. He hides it somewhere (inside a Clay Golem) and then he’s clueless. Now he can’t even remember where he put that egg. The four members of a local band, DCMC, take him in, teach him to play bass, and give him a new name (Lucky). He accepts this new persona and plays in the band, now a five-piece. (Note, the band in Mother2 is called the Runaway Five….) Then Lucas shows up to get Duster/Lucky out of the band to help him, but now they also need to find the Egg of Light again to restore Duster’s old memories. They finally find the egg; it’s in a **Clay Golem that’s kind of busted but keeps fighting and getting away from them**. They finally get the egg out of it at a city dump…. (Side note: Duster’s strongest weapons are his shoes, which can be upgraded later.)
Point of the bonus part? Lizzie is the parallel to the Egg of Light (Hummingbird Egg) and the memories it contains, since she also knows there were twins and wonders what happened to “Ciel’s” memories. She’s also the parallel to Duster’s greatest weapon, symbolized by the focus on her footwear….
Sorry, I didn’t mean for this to be so long, but then I got into “Mother3 mode”…. LOL!