The Argument from Incredulity fallacy is hilariously misused sometimes. Maybe not in this instance, but I don't like it when someone even says that cause it's one of the most overused and misunderstood fallacies in this community. It's what lead to Jiren losing to GoD Toppo in these very forums (which I will never let go of). There's incredulity and then there's common sense.
Anyways I agree it's irrational to assume that Saitama can resist Akatsuki's hax in a versus debate setting, if that's what you're talking about. Cause there's no solid evidence to suggest such, besides maybe Genjutsu but I'm not really caring enough to argue about that, and I wasn't meaning to have a discussion over it in the first place.
But in terms of a story setting, a narrative setting, on no planet are any of their hax gonna do anything to him, he'll shrug it off with the same bored expression, and it's pretty fair for someone to say that considering Saitama's character and what he does in-universe, like it or not.
My point is; Someone saying "I think Saitama would probably be able to resist Genjutsu, Totsuka Blade, etc. cause of how he usually shrugs hax off anyways" isn't nearly the same as "Saitama would be able to resist Genjutsu, Totsuka Blade, etc. cause he just can". Cause clearly the former is framing it as a personal opinion irrelevant to any actual debate or claim. Someone stating their opinion and someone stating a claim aren't inherently the same thing.