I watched some butterfly underwaters, but I think the difference with freestyle is substantial.
- up and down movement
because its symmetric you can pull very wide, push water out and in without disturbing ypur straight line.
If you do this with one arm at a time you are pushingthe whole body out of line.
There is a lot of pressing down on the water to help to get the body out the water, hleping the undulation.
Thats bot needed in freestyle.
So some things are the same, and some are not.
Compare that to freestyle:
Much less forces in non-backwatds direxctions. Everything revolves around the central axis with a touch of bounce and undulation.