It is that easy. I do it (from both sides) all the time. I'm sorry it hasn't worked out that way for you.
This thread exists (for some) because there is an unhealthy belief among some gamers that only WotC can solve their problems, and since that seems very unlikely, said folks are perennially disappointed.
I don't think you getting what I'm saying.
If Matt Mercer published the master swordsman subclass many DMs would allow players to run it
If me or you posted the master swordsman subclass not only would few DMs see it but many of those who do see it wouldn't allow it.
But Darrington Press didn't make a Master Swordsman fighter subclass.