"They dance dances they have made up, to tunes that were almost certainly never used for morris and on instruments sometimes invented in the 20C. They wear costumes sometimes unrelated to the custom they claim to be reviving" - two sentences that contain an obvious contradiction.
If they make up the dances, use non-morris tunes, and design the costumes themselves, why are you assuming that they "claim to be reviving" a custom. In many cases I suspect that they have seen other contemporary sides and decided to produce a variation on the dances that attract them now. They are not "reviving" anything. They are just copying and/or elaborating on a twenty-first-century performance.