If the remedy the court and defendants/prosecution agreed to affects my ability to compete or contract, then I have a claim to that being an illegal remedy, as I was not a party to the suit and therefore did not have say in something that now binds me.
If what I'm doing is illegal, and the court rules that it is illegal, then I don't have right to do it with you (or anyone else), whether or not you like it.
Now, it may not be that black and white. If the ruling comes down to a negotiated settlement rather than a court judgment, then Apple very well may want some say in what that looks like.
If what you're doing is illegal, and the court orders you to stop it and also stop lots of other things not found to be illegal, and also to refuse to allow a counterparty to work with you in even more unrelated fields, then it's fair for the counterparty to insist on having a say.
In this case, the proposed remedy would prohibit Apple from using GCP. Or even buying a Chromebook. Merits of that remedy notwithstanding, I don't see how anyone could have a problem with Apple objecting.
It’s rarely that black and white; moreover, what defines whether is it legal or not is the lawsuit itself. If Apple believes it was not defended appropriately, it would make sense to join suit.
IANAL, but that’s my understanding.