Yeah, I felt a little uncomfortable about my presentation of the exercises for precisely this reason. It was a choice between giving instructions that might be insufficient to cause the desired effect--which is noticing of subvocalization, not subvocalization itself--and causing subvocalization that might not have been there had I a way to communicate "notice subvocalizations" without mentioning subvocalizations.

Still, I'm claiming something whose positive and negative versions are extremely difficult to verify. I'm claiming that subvocalization happens when you aren't paying attention to it, which means you're very unlikely to remember that it ever happened. It's easy to design an experiment to test this, but it would require some expensive equipment few of us have access to. My claim comes from the many many reports of meditators who are almost universally astonished to discover how much narration is going on in their head when all the distractions are removed--and they often aren't told explicitly about subvocalization. Soto Zen is notorious for simply telling students so sit down and shut up, basically. And then they experience this anyway.

Obviously, that doesn't mean everyone experiences it. Just that lots of people do. I'm banking on "lots" being "almost all" for the exercises to work.