I can't speak to Gillberg per se, but my understanding is that diagnostic criteria in psychology tend to be left vague in order to handle less typical presentations of a disorder; doctors are expected to be interpolating not just from the diagnostic criteria but also from the case studies they've read and their previous clinical experience. There's a lot of "I know it when I see it" involved -- which is one of the things contributing to the notorious inaccuracy of self-diagnosis.

I think I'd expect long-form tests like the Wired page linked in the OP to be more accurate than looking at the Gillberg criteria alone, but still flawed; it's pretty easy to let your suspicions or state of mind influence your answers, especially if you have a rough idea of what the symptoms entail.