Forgive me if I'm wrong, but you seem to be coming in with the assumption that most tie-in fiction is by definition inferior to "literature" or even mainstream science fiction.
I would back him up on that assumption 100%. The overwhelming majority of tie-in fiction is inferior to "original" fiction, pretty much by definition.
John M Ford's book The Final Reflection gets a lot of love on these forums. In my opinion, it stands out as the only Trek novel* that can stand with good "original" science fiction. And one of the most striking things about that book is how different it is from other Trek books. It does not use the standard TOS setting and characters, except as a framing device. Its main story examines a whole different culture and that culture's encounter with humans. In one stroke it avoids the generic weakness that limits tie-in fiction, when compared to original sf. And at the same time it zeroes in on a traditional strength of sf, the encounter between alien cultures and values. It takes its place among other great "alien encounter" novels in sf history: maybe not quite LeGuin's Left Hand of Darkness, but not two or three shelves below it either.
Other Star Trek books remain "tie-in" novels. Fun for Star Trek fans, not much to offer non-fans.
* "only Trek novel" -- Obviously I mean, "only one that I've read." I've read fewer than 30, maybe fewer than 20, and only TOS (+ TAS "logs"); so theoretically there are dozens of Star Trek tie-ins I've missed, that are peers of the best original sf novels. I freely acknowledge that theoretical possibility; but I find the odds pretty slim.