Or am I still underestimating human insanity by that much?

Yes.

People dismiss the scientific evidence weighing similarly against them on many issues in the news every day. There's nothing spectacular about finding someone who does it regarding the Earth being flat, especially given that an entire society has existed for hundreds of years to promote the idea.

That you see this as a particularly extreme case of insanity (even in an apparently intelligent, lucid, fully-functioning person) is far more shocking to me than this guy.

Maybe I've just seen too many Louis Theroux documentaries.

At any rate, if it may not be quite a conscious scam, it sounds a lot like belief in belief, i.e. he may be telling himself that he believes the earth is flat because there is no conclusive proof that it isn't, but he secretly knows quite well that the evidence for this is as close to conclusive as it is for anything.