It actually can! Since the RF can see inside your mouth the system doesn't inherently need lip movements and can be fine tuned on what we call "internal speech". Note that this isn't the automatic tongue movements that occur when "thinking" words to yourself, but it's rather like mouthing with your lips closed. We don't often lead with this since it's weird for many people to do, but with a bit of practice, we have been able to get it down pretty well.

Wow that's fascinating. I've added that bit to your text above - I hope that's ok! It answers what was emerging as a sort of mini-FAQ.

Edit: the more that I think about it the cooler and more important this feature seems!

But how would you distinguish all the sounds that don’t require any tongue movement, like (most of?) the vowels?

Vowels definitely require tongue movement! Leave your mouth closed and whisper "a-e-i-o-u" and you can feel your tongue moving.

Vowels change the shape of your mouth's cavity to change which frequencies to emphasize ("formants") and we hear the differences in vowels by analyzing those formant patterns.

There's subtle changes in internal speech muscles as well often for those kinds of movements, and where it does seem ambiguous language context is usually enough to distinguish