This doesn't affect your point, since ObjC does still deal in raw memory addresses, but it has supported garbage-collection since Objective-C 2.0 (i.e since Leopard/10.5)
Yeah, I need a better term for high-level-language than "garbage collected" (I do some Cocoa dev in my spare time, and I do like being able to ignore reference counting. Oh! Reference counting! Another security weakness with Cocoa.)
Not sure how you reach the less secure than C conclusion, unless your claim is that the Cocoa frameworks specifically have a lot of security holes.
Not so much Cocoa's code quality as the worst-of-both-worlds situation it's in with exposure to crappy C code and a very dynamic runtime that is basically impossible to lock down ASLR and W^X-style.