I understand why "just go to the gym" type tips, or just do it for 5 minutes, would work. Because once you're already there you are likely to do more.

But in the gym example you could just leave and you have at least tried. You did something and it's at least progress.

To commit to brushing one tooth wouldn't work for me because I would automatically jump to the future situation of actually being there, holding the toothbrush only brushing one tooth. Obviously I might as well at least run the brush over all the teeth while I'm at that point.

Which is why it wouldnt work. It's too obvious of a trick and could be equated to actually brushing your teeth. There is no real separation between the two.