No one [1] thinks that the human brain is likely to be a digital computer performing arithmetic on fixed-width binary integers and IEEE floating-point numbers. The argument for AI is simply that the mind appears to be a material object and the laws of physics appear to be Turing-equivalent. If that holds, the mind is provably equivalent to a Turing machine (or, less likely, some less powerful class of automaton). Arguing about whether in fact that does hold, as Penrose does, can be justified (though I personally think Penrose lost the plot years ago). Arguing about whether your preferred model for categorizing abstract ideas renders the argument inconceivable, with no reference to experimental results or formalisms, can not.

[1] Feel free to add your preferred qualifiers here. The existence of people who believe tobacco companies are run by lizard-people has no bearing on whether cigarettes cause lung cancer.