Anyone looking for a “gotcha” is missing the point. Cohen’s approach isn’t to expose (no pun intended), it’s to ridicule.
No, it's not. He is obviously not an investigative journalist, but his purpose is to see what kind of reaction he can get out of people. Sometimes the reaction makes people look ridiculous, sometimes it's the exact opposite. The two old Jewish ladies in the synagogue, for instance, aren't ridiculous. On the contrary, they come across as very sincere and actually endearing in spite of the character of Borat.
One of them, Judith Dim Edwards, has died since the scene was filmed, and for some reason, the estate is suing SBC:
Baron Cohen sued by Holocaust survivor’s estate over appearance in Borat sequel (Times of Israel, Oct. 15, 2020)
In my opinion, the scene honors her. And it's important for the message of the film: The world may be full of despicable ********, but they aren't
all ********. And everybody has a choice: to be an ******* or to stand up to the ********.
Judith Dim Edwards appears to have been one of the latter, Giuliani is one of the former, but not because of the shirt-tucking, which appears to be all he's doing.
And no, if your belt is too tight, it's difficult to tuck your shirt in while sitting. You will have to either lie back or stand up to do so.