Syn follows very strict “one finger per fret” except when he has to stretch. The power chord is really just a single note (or three string barre) at position 1, and a 2 string barre at position 3, and that’s if you include the octave – some strictly believe the “power chord” is ONLY root and fifth.
When I started, I used that, but at some point, with my short sausage fingers, I discovered on the lower frets that it was just easier to use index and pinky only, and so most of the time, that’s what I do. Doesn’t change the sound, even though *technically* it’s probably poor form.