I'm not sure what point (if any) gwern is making here but, e.g., Ancel Keys died in 2004 at the age of 100. So, indeed, no wonder he's deceased: he would have had to be staggeringly old to still be alive now.

[EDITED to add:] On the face of it, the most obvious reading of gwern's comment is (it seems to me) something like "ha ha, of course they're dead, because they had stupid theories about diet and health so we should expect them to have died by now". If that is what gwern meant, then I think he was being unreasonable. (But I'm not at all sure it is.)

I totally read it as "oh, no wonder it 'was never fully published' before now, since the lead investigators were the infamous Ancel Keys and a now-deceased collaborator."

Yes. This is not even the first time Keys has been caught dropping or suppressing data in his own studies to support his slant: https://en.wikipedia.org/wiki/Seven_Countries_Study#Early_cr...

Well also, as pointed out elsewhere, Keys championed reduction in saturated fats, etc. to prevent heart disease. If the results of a trial he conducted disputed that idea, that might have harmed his reputation.

It almost feels like a claim that it is safe to put lead into gasoline.

I read it as a joke about the fact that the study is so old they're almost certainly dead (coming from a 21yo 40 years seems like a long time so that could be why I read it differently?)

I read it as a reference to the old idea that "science progresses one funeral at a time". That the information was suppressed because it didn't go along with their opinions, and had to wait until now to come out.

I agree with this interpretation.