As the author points out, there are exceptions to what he's saying (as with most things), but if something is an observation (or stereotype) of a measurable portion of a group, I find that to be of value when taking things into consideration and 'grokking' the world. It does not mean to ignore conflicting evidence.
How would you propose he provide 'facts' to back up what he's saying, When you provide no facts to counter what he's said? Agree with ricricucit that his experience in the field is sufficient facts for me (also helps I have similar experiences in the field, and so concur with his facts)