Dumb is the last adjective I would use to describe Knuth, even if you believe that becoming old makes you dumb, like you clearly do.
My advice to you is to never dismiss anyone's opinion just for being old. And I hope you lose your ingrained ageism before you become old yourself, otherwise you'll find old age intolerable.
Maybe he has seen similar claims before and is too old and dumb to not realize how world changing this is.
My take away is that he views this as another tool we are still figuring out how to use.