Not a disqualification, no; it's more, like you said, that they confound one-another, but usually it's the anxiety doing more of the confounding. It's very hard to determine whether the ADHD really "exists" as its own thing or not until you've treated for the anxiety. (In neurochemical terms, since adrenaline production consumes dopamine, there's no way to know what the person's "normal" dopamine flow is like until you can get them to stop being so stressed out.)

Couple this with the fact that a good few of the psychiatrists I know basically only treat adult ADHD cases—they're easy, no long-term maintenance therapy needed!—and it starts to look more like having anxiety is a "disqualifier". What it really means, though, is that the patient will have to get the anxiety under control before a sensible psychiatrist will trust the calibration of the ADHD self-assessment criteria.