My guess is that although you don't know the race/gender of the person there's a chance that they'll use a different questioning style. More senior/regular users then frown upon this different style not necessarily because they're being racist/sexist just because they don't like the way the question was asked.

Well, in that case you might as well include non-native english speakers.

I'm guessing the point here is that the named groups feel particularly unwelcome at SO; this can be the case without any opportunity for or evidence of discrimination. The blog post wasn't all that clear about how they got the data, but they seem to have data to that effect. It might mean that the expectations of the named groups are different from those of the average white male programmer, or it may only mean that everyone is put off by SO equally but SO is no longer willing to alienate everyone if it also means alienating the named groups.