Just an FYI: That literally means the novels are non-canon.
It's the very exact "type" of canonicity novels always had on Trek: They are "true", as long as nothing on screen contradicts them. The most recent novels are therefore usually always pretty in line with the movies. Hell, many one-off old novels are still "valid", as in nothing has contradicted them, and instead there are even on-screen references to some of them.
This is not a slide. It's just what it is, and IMO one of the things Trek got really right (compared to, say, Star Wars, which wants ALL it's material to be "canon", only to run in into a million different contradictions). For the Trek franchise, it's always the same rule: If you read that novel, and nothing contradicts it, you can hold it "true". Just be warned, that 1) if ever anything on screen appears that contradicts the novel, and that can happen decades later(!) - the on-screen version will trump it. And 2) not many people actually read tie-in novels, so when you're discussing "the adventures of Burnham" online, it's an adventure that most people simply won't be aware of them or treat the contents of them as something "official".
That's how it always has been on Trek, and what they basically confirmed. Star Trek novels aren't so much explicitly "non-canon" (only the real old ones that clash real hard with later material), as they all are "semi-canon": They happened for you, as long as you read them, and as long as they don't contradict any on-screen material. Just don't take anything in them as definite.