They did indeed burn sage and mallowsweet there on the classroom floor, and Firenze told them to look for certain shapes and symbols in the pungent fumes, but he seemed perfectly unconcerned that not one of them could see any of the signs he described, telling them that humans were hardly ever good at this, that it took centaurs years and years to become competent, and finished by telling them that it was foolish to put too much faith in such things, anyway, because even centaurs sometimes read them wrongly.
Harry Potter and the Order of the Phoenix
In my opinion, it should use the phrase "none of them", but I might be wrong. I'd like to ask if there's any difference between "not one of them" and "none of them". Any thoughts?