I suspect some significant percentage of applicants may not have a disability, exactly, or always totally flub an interview in this way, but may do so often enough that it looks like the level of competence in the industry is much lower than it actually is, if one is taking one's personal experience with interviewees as an accurate measure of that. Add to this that assuredly some of the people confidently complaining about how 90% of their applicants can't write a for loop overlap with the ones generating complaints from applicants that some interviewers are themselves incompetent and asking broken questions (I guarantee you the people doing this think they're great interviewers and getting nothing but signal from their process, and they're probably also likely to exaggerate stories when relating them), then factor in a real tendency to take a 90% OK-to-good signal in interviews and practically forget it happened, taking the 10% bad as more accurate, and I think it's highly plausible the state of things isn't nearly as bad as some believe it is.

Tech interviews are remarkably scattershot in the form they take (outside well-known big companies), and are unusually anxiety-generating, even in the notoriously anxiety-filled field of interviews. Describe what one might (emphasis on might, part of the problem is that it's so often a surprise) expect in a tech interview process to some people outside the industry, and gauge their reactions. I definitely think it's likely they have even worse signal-to-noise ratio than is commonly thought.

[EDIT] Certainly I find it far less plausible that there's an absolute army of people out there, dwarfing the count of actually capable programmers, who are brilliant con-persons but too dumb to figure out that that skill itself is more valuable than programming, outside the top couple percent of programming jobs by comp, and apply it more directly to business roles that actively want it.