Setting Google as the default search engine had the same effect as pre-installing Microsoft Explorer on Windows. If anyone knows how powerful that default setup is, it's Microsoft. I lean Microsoft on this one even though they have been a bad actor in the past.
Whether it was an antitrust violation for Apple and Google to agree....that requires info we do not have. That being said, those two companies have a history of entering into anticompetitive agreements (e.g., their non-poaching agreements).