I've heard (but don't have any actual evidence more than hearsay) that Twitch generally operates independently of Amazon/AWS. I'm sure that they share some things, but I wouldn't be surprised if their source was separate from the "main repo"

Remember that Amazon runs one of the biggest multi-tenant service platforms in the industry! A separate business unit like Twitch is likely to be set up a lot like any other random AWS customer, and you wouldn't expect that compromising servers used by one AWS customer to automatically compromise the underlying infrastructure.

(I would also expect that the Amazon retail systems are in most senses "just another tenant" on AWS, albeit with much more liberal quotas!)