Yes, they had GC working with Objective-C but there were so many problems with it that they dropped the GC in favor of ARC years ago. By the time Swift came along, GC with Objective-C was no longer an option.
Chris Lattner was already working on Swift when the decision to drop GC was made. Guess who made the decision? Chris Lattner. If anything, GC was dropped because of Swift, not the other way around.
