From Ammoland:

In particular, one appellate judge was very concerned that, in the future, the “ATF could choose to redefine ‘machine gun’ as including all semiautomatic weapons that can be modified with a device like a bump stock.”
Of course, this is exactly the point that GOA has made in its briefs to the court. If the ATF can claim that a bump stock can turn an AR-15 into a machine gun, then the same can be said about rubber bands or belt loops.