I get was DW is saying. Jeri Taylor considered some of her elements of her books canon (I don't believe this was ever actually the case--correct me if I'm wrong but I don't think anyone at Paramount at the time said, yep, it's just as legit as on screen canon).
Contrary to fan myth, canon is not some official doctrine handed down from on high. It's just a nickname for the core body of work. The creators of that core body don't have to say it's canon, because that's what it is by definition. The only time it's a question that needs to be addressed is where tie-ins are concerned, because canon fundamentally means "the stuff that isn't tie-ins or fanfic," but there are occasional exceptions or gray areas.
The label is not more important than the thing it describes. Putting a label on a thing doesn't make it what it is, it's just a convenient way to refer to it. "Canon" is a convenient way to refer to the overall, consistent body of stories that the creators of a series put out. Jeri Taylor was the showrunner of Voyager, the one in charge of telling those stories and shaping their continuity, and since she also wrote those two novels and considered them part of the same whole, that made them part of the canon that she was creating. The label didn't cause that, it just described it. But once other people took over creating the canon, they chose not to be bound by her books. Canons are mutable things, and they often exclude or retcon things that they once included (just ask Bobby Ewing of Dallas).
So in a sense I think you're both right. I see greater continuity between all elements of Discovery as DW suggests, but it doesn't make the novels or comic books canon as Tuskin noted.
Yes. The label doesn't matter. It doesn't make things what they are. It's just a nickname. If the show stays consistent with the books, that's not because some stupid little 5-letter label imposed some kind of magic power; it's just because the people making the shows chose to stay consistent with the books. What Sullivan said is just a way of saying that the producers are aware of the books and have no current plans to contradict them. That is a change in the usual approach, but it's not like someone passed a law or something. For that matter, we don't even know if the other producers agree with Sullivan's take on the issue. We might get a "clarification" from Kurtzman or Berg & Harberts tomorrow saying the books still don't count.
And of course, to the extent that it does apply, it only applies to the Discovery novels, the ones that Kirsten is riding herd on. The reason canon usually excludes tie-ins is because the creators of the core work don't have time to be aware of everything the tie-ins are doing. The only canonical tie-ins are ones that are directly supervised or written by the same people who create the original series. The DSC novels and comics pretty much fit that description thanks to Kirsten, but the other Trek tie-ins don't.