I think a spinning mirror would make it a lot harder. He's only moving the mirror after the "animation" finishes. So it's capture video, step by 1 pixel, capture video, step by 1 pixel, capture video, etc... He's replaying the scene ~1 million times, for 1 million unique single pixel 2 billion fps videos.
Basically the constantly spinning motor would remove the complexity and slowdown of accelerating and decelerating it at the start and end of the motion. To capture a line, he's already scanning it across the scene and triggering the laser at specific points (when the encoder in the servo reaches certain values), if it was continuously rotating he could just do the same thing.
Though I don't think it would speed things up much, from what he was saying in one of the appendix videos on his second channel he doesn't do things like triggering the laser multiple times for each pixel to reduce noise because the bottleneck is copying the data off of the scope and it would stretch from hours to days to run.
