"open source" and "in the public domain" aren't like separate things from "copyright", they describe specific sub-sets of stuff underneath "copyright", which is a top-level category that establishes a meaningful definition of stuff like "your work(s)" in the first place

i.e. "copyright" describes a legal concept, "copyleft" describes a licensing concept

The point is the legal concept is a fiction, useful perhaps in the past, but increasingly not today, and like all fictions, it can be dispelled if people agree to do so.

i mean sure if you're gonna take some epistemological position then every legal concept is a fiction, but that's not a particularly interesting nor useful angle on the discussion

or, to put it more succinctly, and quoting one of the great contemporary aphorisms: sure, and if my grandmother had wheels, she would have been a bicycle