He also points out (just a few words from the bit you quote) that virtually everyone registered with an email address that included their name, so he knew who everyone was.
Surprisingly high percentage, but I guess in such a relatively large group the negative effect of someone from that group having access to everything isn't as large as it would be in a smaller one. I don't think that would have working in my highschool, at least not without encryption and source code available...
"Hunter ID number (a number which I could validate but not easily trace to a particular person)"
I'd read that as some minimal formatting and the correct number of digits and you're in.
I suspect there were no small number of school admins, teachers, parents, ex students, middle/grade school students, outright adult pedos, etc.
More like 500 registered users out of a billion internet users.