Agreed. As I've had preached at me

, canon is anything that appears on screen.
Rather, canon is the original creation as distinct from its imitations. In the case of a franchise like
Star Trek that began onscreen, the original stuff is onscreen and anything offscreen is a tie-in -- just as in the case of a franchise like
The Expanse that began in books, the canon is the books and the films are adaptations. Sometimes a canon can exist in more than one medium -- for instance, the canon of
Harry Potter is in books, but the
Fantastic Beasts films are original works by Rowling and thus are canon in themselves. And some canons are multimedia, like
Babylon 5, Buffy/Angel, Firefly, Star Wars (theoretically), and
Avatar: The Last Airbender/The Legend of Korra.
What makes something a canon is not its medium, but its authorship. The common thread among canonical tie-ins is that they're written by or supervised by the creators of the original work and thus are of a piece with those creators' vision. This is why the
Babylon 5 novels and
Buffy comics that came out during their respective series were not canonical while the ones that came out after the series ended were canonical -- because only in the latter case did the creator of the series have time to supervise the tie-ins and make sure they were truly consistent. Also, a series in progress is a moving target, its ideas constantly changing, so even the most informed attempt to keep pace with it can end up getting contradicted. To have a consistent canon, you usually have to have only a single thread of it developing at a time. (
Star Wars is trying to have multiple parallel canonical threads, but the movies and TV shows have already contradicted some of the supposedly canonical books and comics in some ways.)
A lot of people misunderstand "The canon is what's onscreen" to mean "every last onscreen image, word, and detail is immutable fact," but that's nonsense. Canon is the whole, not the parts, and it's not about granular-scale "reality," just about the pretense of telling a continuing story, even if the details sometimes get tweaked along the way.
Really the only ones that need to even worry about canon for Star Trek are tie-in authors. Each franchise is different, in some the tie-ins might carry more weight. But in the world of Star Trek, the tie-in writers have to stay consistent with what's on screen (which in Star Trek is considered 'canon' these days).
You're confusing two different directions of influence. It's routinely the job of tie-ins in every franchise to stay consistent with the series they're adapting. That's the whole point of the exercise, to create something that feels like it's part of the series. But that almost never implies that the reverse is the case.
And I wouldn't call it "worrying" about canon. Again, "canon" is just a nickname for the original work as a whole. We don't "worry" about being consistent with it, it's just the definition of the job we're trying to do. Fandom has this delusional idea that "canon" is some kind of official seal of approval that has to be earned through some kind of purity test. It's really just a descriptive term for one thing as distinct from another thing. You don't have to "worry" about it any more than you have to worry about whether something is land or water. It just is. (Well, I guess you would have to worry about that if you were, say, running out of gas while flying over the ocean. So it's an imperfect analogy.)