That's only changed about three or four times in the past forty years. But the DM hasn't primarily been the ref since before Gygax left and the Dragonlance modules ruled.
5th edition DMG, page 5: "Dungeons & Dragons isn't a head to head competition, but it needs someone who is impartial yet involved in the game to guarantee that everyone at the table plays by the rules. As the player who creates the game world and the adventures that take place within it, the DM is a natural fit to take on the referee role." and "As a referee, the DM acts as a mediator between the rules and the players."
4th edition DMG, page 12: "the DM does act simultaneously as the player controlling all the monsters and as the referee. Being a referee means that the DM stands as a mediator between the rules and the players."
3rd edition DMG, page 9--and 3.5 edition DMG, page 6--gives probably the most powerful explicit definition of a DM's authority: "When everyone gathers around the table to play the game, you’re in charge. That doesn’t mean you can tell people what to do outside the boundaries of the game, but it does mean that you’re the final arbiter of the rules within the game. Good players will always recognize that you have ultimate authority over the game mechanics, even superseding something in a rulebook." Page 17--18 in the 3.5 book--"You're the arbiter of everything that happens in the game. Period." and "The DM really
can't cheat. You're the umpire, and what you say goes."
Granted, the second edition DMG is really bad at defining the philosphical role of the Dungeon Master, but for more about 39 of the game's 50 years, the DM has been a referee. I'm not suggesting it's the DM's
primary role--at least I think primary was your word, not mine though i could be wrong--but it's been A core function of the DM for most of the game's history, non consecutively, and the idea definitely didn't go away for ever when Gygax left TSR.