I think Hislop genuinely doesn't get it, a bit like other supposedly intelligent men such as Rory Stewart, Alastair Campbell, Philip Pullman, Jon Ronson etc.
Part of the reason is these men don't understand why women, particularly vulnerable women, should be frightened of men. They probably don't care either. Why would it bother them if a man occasionally gets undressed in a female changing room, or a male criminal is put in a women's prison? In fairness there are a lot of middle-class women who think like this too. They lack imagination.
The other, more interesting, reason, to my mind, is an inability to engage in mathematical reasoning. It's absolutely no coincidence that two of the most prominent critics of gender ideology are Helen Joyce (PhD in maths) and Kathleen Stock (professor of philosophy). People who can't think mathematically decide that because only a "minority" of people identify as trans, the harm they do is minimal. And because the majority of trans people aren't sex offenders, therefore we're only talking about a minority of a minority of people, which is hardly worth getting upset about.
What they don't understand is that, once you allow people to self-identity, any man can identify as a woman. In mathematical terms, the set known as "women" can, in theory, expand to include all women and all men - in other words, everyone. And of course the men most likely to take advantage of this are going to be men who intend harm to women: everyone from flashers and knicker-sniffers to paedophiles, and rapists.
It's tempting to blame our education system for churning out people who can get a degree in PPE or humanities from Oxford, but can't engage in very simple-level mathematical thinking. However, I'm not sure it's entirely the fault of the education system - I think some people just can't do that kind of thinking. It's an inherent failing in their brain, a bit like being dyslexic.
The bigger problem, unfortunately, is they don't realise that they have that failing, and imagine that because they have their posh education, they must be clever - thus enabling them to look down on people like JKR who is, frankly, several leagues above them intellectually.