He's full of it here, right? He doesn't make it clear he's joking. The projectionist part is obviously meant to be a punchline, but the rest of it...?
He makes it sound like a bunch of professional critics found "Three Amigos" so reprehensible that virtually all of them violated their codes of conduct and departed the screening before it was over. Actually, he doesn't say professional critics, just critics, so maybe he has some wiggle room here. The main thing Reed wants to do is be funny and trash the movie, and I have to admit he does it in a way that gets my attention.
I have said it before, and I will say it again: The more I see the Siskel & Ebert imitators, the more I appreciate the originals and how they could pan even popular movies but still be likable.
(For what it's worth, in their print columns, Ebert gave the movie one star, and Siskel called it boring, but neither claimed to have attended a critic screening at which everyone else left early.)