>But who is "they"?

The person you're replying to.

>What does it mean to be "on the same side as Rust"?

There is a stated and auctioned stance the rust foundation poses, and they are saying they align with that. At this point if you can see their point the communication fault is yours.

>Being on a side suggests an unwillingness to part ways

No it doesn't.

>If Rust is a worthy rock in the political sphere for the moment

I have to assume their politics is more intimate to them than the maintainers of a programming language, and that they used it here because the conversation we are having is around rust.