I don't doubt the author's credentials, and the program is probably correct (I don't care enough to check). But if you write a program and tell me it does what

I don't doubt the author's credentials, and the program is probably correct (I don't care enough to check). But if you write a program and tell me it does what you say it does with a high level reason why you think so, that's not a "rigorous" proof. It's a sketch of a proof.