Obviously the SSC readership is a very long way from being an unbiased sample of (say) the whole world's population. No one would expect it to be, and in fact that's the point here: a survey of people who are obviously unusual in some respects (whatever combination of quirks turns someone into a likely SSC reader) turns out to be unusual in another respect with no obvious connection, namely having substantially more firstborn children than you'd expect.
Whatever it is that makes someone more likely to read SSC, it seems like it's probably a combination of things that surely can't correlate with birth order (e.g., being a native English speaker) and, broadly speaking, personality traits (e.g., being interested in the sort of thing Scott writes).
So the results show evidence of a link between birth order and personality, and (from the survey results) apparently a strong one. Which is interesting if true. And all of this only works because the SSC readership is far from typical of the population of a whole.
So, again, what do you mean by "skewed"? And why is it a problem?
Sure, who's claiming otherwise?