I think some amount of not-yet-done research would have to go into proving whether or not they're true leucistics. It's my suspicion that they are, but the question of Super Fires is an interesting one. In that case, some skin cells do seem to be able to support at least certain kinds of pigmentation. But if that were to be true for Lessers or Butters, I feel like we would have seen it by now--thus I think it's safe to call them true leucistics.
PS, I didn't get to finish my incubator before I left, but I'll be bugging you for some help in January if that's okay!
Cheers,
-Matt