Is this just his speculation? Or is there evidence for it?

What the instruction does is well documented, and the history of other invalid instructions being used for the same purpose in the past I'm guessing there is also well known, though a quick search doesn't turn up any official Intel documentation on the matter.

Given who this is and the overall quality of his output over the years, I'm willing to trust it isn't pure guesswork - and anyway, I'd trust his guesswork over many other people's absolute facts.