This is not legal advice.
That much is clear, because it would be horrendously bad legal advice if it were. You went out of your way to quote a ToS that explicitly gives Twitch far more rights than you claim they have including derivative works, and somehow translate that to Twitch only being allowed to transcode video?
With that we can only assume that:
a) you don't know what all the words in the ToS mean
b) you don't know that courts already have ruled that AI training is transformative and fair use implying that the only question is whether content was legally obtained which in turn means that so far as every court has ruled to date, if you publish something for free or upload it to a service, it is in *legal* fact, fair use for AI training.
By their own admission, they had to force everyone to opt in because nobody would have if they didn't force it.
No, by their own admission the mechanism to opt-out was disabled by default. Based on current legal precedent and given the ToS you quoted, they actually don't need to give people a choice at all. But when they did give that choice, they made it opt-out, which ... of course they did, they would have been mad not to.