It's not binding because the other party hasn't agreed. You agree to terms when you use the site. One party can't unilaterally change the agreement without consent of the other party.
I see where you're coming from but it's not quite the same thing; Facebook doesn't encourage people to choose a license for the content that they post there, so there's no expectation that there are any terms aside from those in Facebook's ToS. OTOH GitHub has historically very strongly encouraged users to add a LICENSE to their repositories, and also encouraged users to fork other people's code and and push it to GitHub. That GitHub would be exempt from the licensing terms of the code pushed to it, except for the obvious minimal extent they might need to be in order to provide their services, seems like an extremely surprising interpretation.
It has nothing to do with GitHub being exempt from anything. It's that users are bound by the terms they agreed to in a ToS. If there is a conflict between a user-created license and a site's ToS, the burden is on the user to resolve it.
To be clear, I'm not suggesting this is some kind of loophole GitHub is using to trample on users' licenses, even though maybe they could. It's probably completely legal for GitHub to use even the most super-extra-double-GPL-licensed code because copyright law allows it.
The author of the Twitter post's suggestion that Copilot's output must be a derivative work is based on a naive understanding of "derivative" as it's defined in copyright law. It's not hard to find clear explanations of how this stuff works, and it's obvious she didn't bother to do any homework. Several criteria would appear to rule out GitHub's use as infringement. e.g.:
'In essence, the comparison is an ad hoc determination of whether the protectable elements of the original program that are contained in the second work are significant or important parts of the original program.'
