Yes, I think that's a good piece of feedback to provide, and would have been fine as a direct response to the topic. However, IMO your first response was too dismissive given that the thread was indeed on-topic.
Most users' first exposure to Swift (right now, at least) will be through an Apple framework, and so chances are many beginners' questions are going to mention or import some Apple framework. If the knee-jerk response to such questions is, "Please don't post this here," the forums are going to be an unwelcoming place for beginners.
If you are telling users that their on-topic posts are actually off-topic, that is not helpful and people are going to be upset. I have not seen significant community pushback on users telling topic authors that their actually off-topic posts are off-topic.