This seems to be a quote taken out of context. Apple wasn't against third party frameworks as a rule — plenty of third party libraries and tools popped up, especially for allowing languages other than ObjC to be used effectively on iOS.

The message in Thoughts On Flash wasn't about third party libraries wholesale; it was refuting the expectation that Apple should special-case Flash, a technology that never ran acceptably on Android and was quickly deprecated, never mind on iOS.