this is correct, and this is why i often say that virtually no one (not even SwiftNIO) really adheres to semver.
as a practical matter, i've just resigned to ignoring the problem and assuming that extension conflicts with nested types simply won't occur. but name conflicts with toplevel symbols can and do happen quite often, and this is something i've been burned by many times using other peoples' libraries.
so, as a general best practice, nest everything.