That the patent exists and was legally acquired is not up for debate.

I should have asked: Does Apple's license make it harder for someone else to challenge the patent?

I assumed it would, but if I'm reading MedImmune v. Genentech correctly then it doesn't even make it harder for Apple to challenge.

No one is questioning the existence of the patent - it's the validity of the patent that's being doubted. Given the current processes followed by the USPTO, existence says _nothing_ for its validity.

Apple actually has a license for the patent. So yes, but in a much more direct way than you suggest.