Edited to amplify: I have never seen the term previous to this thread. Google doesn't turn up much beyond the quoted quip. Is ironic sincerity when you pretend to pretend not to believe what you're saying and then everyone pretends to pretend you didn't believe it so that no-one need be put to the trouble of thinking about it and deciding whether it actually made sense or not? Or not?
It means that it was a true statement, but that reading the statement still tickles the "irony" feeling in your brain.
I think part of the reason that this is so is that some people sympathize with this mathematician's motives. An analogy:
"He donated $1,000 to charity, instead of donating his entire discretionary income."
"How utterly selfish of him."
It's true that it's selfish, but it's a lot less selfish than what most people do, so it feels ironic and sarcastic that we are calling him selfish.
A monthly thread for posting rationality-related quotes you've seen recently (or had stored in your quotesfile for ages).