I don't think this is broken by a Day 1 massclaim.
It is, however, almost certainly broken by a Day
2
massclaim (including night action results). I don't think that outright proves anyone's alignment, but it makes it very hard for scum to come up with a convincing fakeclaim because they don't know which of the town claims is which (and for that matter, neither do the townies).
I think an interesting direction to go in with this sort of Dethy variant is to tell the scum the entire setup in advance (including who has which role), in order to give them the best possible ability to make plausible fakeclaims. At that point, though, I have more or less no clue where the balance lies, although there's a distinct possibility it would be scumsided.