If Twitch helped to cover up a criminal act, they'd be opening themselves up to liability. Especially since they supposedly provide the product used for the communication AND apparently knew about it.

The simpler answer is, the reality isn't as simple as the tweet makes it out to be. Twitch may have thought/known the user was a minor based off internal-only information, like previous messages, account information, etc. not anything in the conversation with Doc. In that case Doc would not have known they were a minor, and thus his actions would not have been illegal, and it would not be a story at all if Twitch reached out to advise Doc that the user was a minor... instead Twitch acted unilaterally and essentially burned the contract in the process. That would fit the same "facts" we've been told from all parties, but with a vastly different context that also matches the lack of criminal liability.