They are not a separate continuity, because the screen continuity is already part of their continuity
And this is where you are fundamentally mistaken. Spin-offs don't control the canon. Your argument is that CBS licensing and Pocket Books actually controlled Star Trek continuity because they've made an expanded one (or "ones") that expanded on, referenced, and interpreted the live-action works.
That is wrong, and is part of why acknowledgement of rank is so critical to properly understanding things. Just as you shouldn't accept the words of the school janitor as the final word about what is or isn't school policy, so it is that you must properly recognize who's in charge of what with respect to Star Trek.
Back in the day the seat of Star Trek power was in Rick Berman's office. Now it is in Kurtzman's. Both are outranked by assorted corporate suits, to be sure, but this is generally true.
The ranking of continuities follows the same logic. Pocket Books and the gang at licensing may have their own little 'overall continuity' going but it doesn't control the primary production continuity any more than someone's headcanon which might include a fanfic can control the Pocket Books continuity it includes as part of a 'super-duper-continuity'.
That is too say, you are arguing for this:

But because you're ignoring rank you don't recognize that this is just as valid, by your argument:

Monday and Tuesday on screen, Wednesday was a book, Thursday was on screen again.
Don't you understand that by adding a new story about Wednesday you just changed the whole week?
(Of course, that pretends that the books are so narrow in focus, which is seldom if ever the case.)
What can happen here is...everything between Nemesis and the new Picard show happened in the books, then the new Picard show becomes the Thursday in my example. (Unless of course they do adapt Destiny. Then it becomes retelling of Wednesday.) It does not transmite into a third continuity, it is simply a continuation of continuity already in place for a number of years.
You are merely saying that the existing Destiny continuity can be overwritten, but missing the point that it exists as a thing that would be changed.
Similarly, Picard had a future in the live-action-only canon, whatever it was to be.
Now this new mixed continuity where Picard has a different past that includes books and comics events will write a different one.
That's Tuvix, in a nutshell.
I have already established how easy it is for STO to adjust itself to the new approach.
So? STO isn't referenced by Kurtzman.