It probably wouldn't hold up in court, but it can be used as a bludgeon to dissuade people from filing in the first place. Roku is totally allowed to lie and say "You can't sue, you agreed to mandatory arbitration. // You can't join the class action, you agreed not to. If you do either of these things, we'll sue you."
This could easily dissuade quite a few people from litigating, limiting how much the company needs to pay out.