Where the check did you hear that cock-and-bull story? The actor who plays Jerry/Jimmy Stilled is most definitely NOT veteran comedian and Frank Costanza portrayer Jerry Stiller; it is, as credited, longtime character actor Scott Wilson. My guess is that the character was called Jerry in the script, but they got a good take on a tough scene where he was incorrectly addressed as Jimmy so they stuck with it. Or something along those lines.
But if it makes you feel better, Wilson played Boo in "The Heartbreak Kid" which featured Jerry Stiller as the main character's father (who was played by Ben Stilled).

I thought so! I mean, that you saw it on THIS TV as well.
If you put me on ignore, then how can I notify you when I win the lottery?

The actor playing Jimmy/Jerry Stiller is this actor:
http://www.imdb.com/board/20005467/reference
Jerry wanted the part to be uncredited because he realized after he agreed to do the part that it wasn't big enough to have his name listed in the credits. So Jerry had the makeup artist make him look like Scott Wilson. His character's name was Jimmy Stiller but someone who submitted the character names goofed and used his real name as the character's name.

Everyone does call him Jimmy in the movie. (8 times)
But the end credits have the character as "Jerry Stiller."
They goofed up.

I believe Mr. Fart is the correct term

correct

I just watched the film, and the end credits do say
Jerry Stiller . . . . . . . . . . . . . . . . . . . SCOTT WILSON
"Be Quicker of Mind Than of Tongue"