I don’t see why not, but there wouldn’t be any need since belt fed weapons are almost always full auto anyway. In any event it seems to work with a bipod.
If you study it you’ll find that the RPM can vary because of how you hold it with the bump stocks. If you let your shoulder or body deflect to absorb some recoil it slows down because it’s not going into the stock. Pure Physics.
Considering his reputed wealth and overseas connections it’s easy to see him getting hold of a light machine gun. But so far all we are hearing about are the bump stock guns. Did it sound to you like the RPM varied? I wasn’t listening for that so I have no idea.