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.)