The key to mass arbitration filing is that it's the lawyers doing it, and they're the ones showing up wherever in place of the people who have signed agreements with them. Said lawyers are essentially gambling that the target company will give up and allow normal legal maneuevers, because doing arbitration en masse is actually really expensive for the company mandating arbitration.
FWIW I’ve started to see new arbitration agreements pop up that specifically try to disallow this strategy by using verbiage like “ only the consumer is allowed to file this claim” and even “lawyers who would handle more than 20 of these claims are not allowed to file them”. I have no clue of the legality of such clauses but I have seen them in the wild in the last few months
