It wasn't possible until Swift 5 because of ABI stability. If we were to do that before Swift 5, we would still have to create binaries for each version of Xcode/iOS.
The ABI stability in the article is solely provided by ObjC, not by Swift. The "Swift ABI" of 5 only helps you with not having to embed the Swift libraries within the ObjC shielded module. (it "just" makes it smaller)
The point of the article is that you can use the stable ObjC ABI to wrap the Swift incompatibilities. And that was technically possible since Swift v1.