This is what you are missing.

No, I don't think I'm missing this piece. The claim is very general: ALL "neurologically healthy people".

People can certainly be mistaken about matters of fact. So what?

It's not reasonable to take people's claims that "by 'should' I mean that X maximises utility for everyone"

Of course not, the great majority of people are not utilitarians and have no interest in maximizing utility for everyone. In normal speech "should" doesn't mean anything like that.