Unless something very official comes up someday, I still believe that these X-Ships are and were CGI-Screw. Nothing more and nothing less. We all have our own opinions and believes, whics are all 100 % real in our own minds. As always, that is just my own opinion.
Though I find the theory of "experimental testbed ships" fascinating, given the fact that Earth had quite an interest in "Hybrid ships" like Omega-X, Shadow Hybrid Vessel, Warlock and later Victory-class models. But all of these ships were created after a war, so they will not make this theory any more "official" than any other.