I don't agree. Many professional authors have written professionally published works in someone else's universe. There have been 100s of Star Trek novels by dozens of authors going back to the 1970s. There are fewer Star Wars novels, but still a significant number of different authors. Including authors who had been commercially successful with their own works before writing Wars. Such as Timothy Zahn and RA Salvatore.
Robert Jordan, who you may know wrote a major fantasy series, one that lived on after Jordan himself died, many of his successful early published works were Conan the Barbarian novels. Robert E Howard started that series in the 1930s. Other authors who published in the series include Poul Anderson, Leonard Carpenter, Lin Carter, L. Sprague de Camp, Roland J. Green, John C. Hocking, Sean A. Moore, Björn Nyberg, Andrew J. Offutt, Steve Perry, John Maddox Roberts, Harry Turtledove, and Karl Edward Wagner.
Valka has talked in the past how successful fantasy author Mercedes Lackey got some of her start writing fan fiction for Marion Zimmer Bradley's Darkover series. Lackey, as well as many other authors who have successful series, has anthologies of short stories published in the universe of those series. And most of those short stories are little more than fan fiction itself. Just fan fiction which has gained a commercial publisher. Eric Flint, the author of the popular fantasy series 1632, has had the universe he created there essentially taken over by fan fiction. I've been trying to get caught up on the series, but Flint himself says that to be really on top of what is going on you have to read all the published volumes of what is, in effect, fan fiction. At the same time, Flint changed the direction, somewhat, of David Weber's Honor Harrington series when he coauthored a couple of the books, and wrote some of the short stories for the anthologies on that series.
Years ago I read a series of books called
Thieves' World. This was explicitly a project where the editors created a world for other authors to write short stories in. And often some of the authors would @$&(*^ with the characters created by other authors.
In short, shared universe stories are nothing new, and nothing out of the ordinary.