What dune calls virtual libraries and implementations is nothing new. They have existed in the eco-system for almost as long as ocaml has (e.g. the threads and (now gone) vmthreads libraries).
What is new is the ability to automatically select implementations via a variant tag which you can see as icing on the cake. This is not supported by ocamlfind nor by my proposal (I don’t think it would be hard with more metadata, but I also think it might be a bit premature).