If you already think he believes something for a reason you believe is wrong, you don't have a very high opinion of his rationality.

If your goal is to improve, it's more important to notice and correct errors than deceive people about their absence. I believe it's insulting, not respectful, to attribute to a rationalist the attitude that they would prefer the knowledge of a flaw withheld.

(You might want to take a precaution of asking first if Crocker's rules apply, and communicate the bug report privately.)