Looking at the response, Hussie actually doesn't outright say it's biological; what he does is make some logical extrapolations from what's already given in the comic for how long each caste's lifespan would probably be, and then state that he's not going to give any solid answers. Nowhere does he say *why* there's a lifespan difference. "The Condesce seems to have a lifespan that could naturally be hundreds of thousands of years, or perhaps millions, even without LE’s powers." I note 'seems'.
Oh, this is encouraging. I still haven’t looked at the thing, actually, but yeah it’s more likely to have been sort of vague and misleading. I would have remembered otherwise, as I was actually around for the Q&As in 2012.