Side note, I think it was hilarious that Swift was allowed on FOSDEM. Even “free” (as in you probably don’t have to pay for a developer account to use it, *unless you want to ship some binary), Swift remains an Apple product.
You need one only to ship apps on Apple platforms, but that's unrelated to Swift. It applies also to apps written in Objective-C, C/C++, and multi-platform language/frameworks like Dart/Flutter.
Deployment of any software (unrelated to Swift) on Apple's platforms is entirely unrelated (and even then, at least on macOS you and any other user can install, sell, buy (...) any binary as desired).