If I recall correctly he is in academia dealing with some massive questions, I don't think he has put a lot of emphasis on how his website is constructed or maintained, as long as its readable to the public.
Hmm, I think some people on Hacker News are in too much of a bubble giving no thought to the priorities of others. Not everyone builds web apps. Sorry dude just seems like such a silly thing to comment on. I'll get off my pedestal now. :)
For those saying that Stephen Hawking deserves a free pass, if we apply different standards to different people then that's not morality, it's just convenience. And anyway I doubt it was his idea to post that notice, more likely a misguided publisher.