The Reddit post you talk about, I think, just means someone installed the game files, which wouldn't have been hard to get anyways very likely. I don't think it says anything important here
As for your question, I'm not a dev or anything but I don't think Plutonium devs are interested in adding new games, even less non COD ones
I don't have the technical knowledge to tell you if a client could even be possible for XDefiant but what I know for sure is that it may have a small community with very limited content and possibilities, which might not motivate people to work on it in the first place. The game didn't even have time to get people to love it enough to have modders consider wanting to keep it alive, in my opinion
Clients happen on these old CODs because people have passion for them, with years and years of good memories etc
XDefiant was just a few months beta that went away
