1. MOTU started life as toys. In an attempt to give the toys some life, Mattel included mini comics that provided some backstory to the characters. Early on, He-Man had no secret identity, had no relation to the king, his strength came from his harness, etc, etc, etc. Although at times hokey, the tone for these early stories was darker than what it later became.
2. Filmation turned MOTU into a cartoon to help sell the toys. When this happened, they threw most of the early mini comics stories out the window and created their own. He-Man gained a secret identity, his power was linked to his sword, "By the power of Grayskull!" was born, etc., etc., etc. The tone become much lighter and the villains (the flunkies, anyway) became uniformly dumb and absolutely no challenge to He-Man. I can't even count the times poor Beast Man literally got tossed.
3. Comic books then became something in between the early mini comics and the cartoon series -- they kept a few things that became established in the cartoon, but they didn't exactly follow the continuity established in the cartoon.
4. As the years went by, the mini comics became lighter and their quality oftentimes suffered tremendously. None of these mini comics, regardless of when they were created, followed any particular canon (even amongst themselves). It's as if they lived in their own worlds.
5. Picture books came out... again, canon out the window.
6. The 1987 Dolph Lundgren movie ignored He-Man's dual identity altogether (presumably taking a cue from the early concept of the character), gave us never-before-seen characters (Karg, Saurod, Blade, and Gwildor), and a much nastier Skeletor than anyone had ever seen (and despite what anyone thinks of the film, Langella's cruelness was excellent).
7. NA didn't exactly fly against the face of earlier Filmation continuity, but they did put He-Man and Skeletor in a completely different time/place and it did give us a version of Teela that looked a lot more like She-Ra than the redhead we'd all been used to.
8. Mike Young gave us the new 200X cartoon, which basically gave MOTU a darker tone (for a cartoon, anyway), brand new backstories for most characters, and a much more cohesive continuity than any of the previous incarnations.
9. And it was essentially the same for POP. The Filmation series did not match the mini comics that came with the toys (not to mention most of the toys looked nothing like their cartoon counterparts) or the picture books/comic books.
So this very long-winded reply is simply to state that canon is not and never has been sacred with this brand. Each iteration the brand takes gives its own backstory. Yes, Filmation had Shadow Weaver and Castaspella as early rivals, but that doesn't mean this incarnation of the story must follow that. In all honesty, they never even kept that up on the POP cartoon. They only did it for one episode and their supposed "rivalry" never came up again. So, I for one, don't mind if they choose to do it differently here. And who knows? Maybe Castaspella is from the past as well.