I have actually read it and agree it is impressive. I will not comment much on the style of the writing, since this is very much subjective, but I would rate it as the "typical" modern fantasy style, which aims at filling as much pages as possible: very "flowery" language, lots of adjectives/adverbs, lots of details, lots of high-school prose ("Panic was a luxury they couldn't afford"). Not a big fan of that since I really miss the time where authors could write single, self-contained books instead of a sprawling series over thousands of pages, but I know of course that this kind of thing is very successful and people seem to enjoy it. If someone would give me this I would advise them to get a good copy editor.
There are some logical inconsistencies, though. For instance, when they both enter the cellar through a trapdoor, Kael goes first, but the innkeeper instructs him to close the trapdoor behind them, which makes no sense. Also, Kael goes down the stairs and "risks a quick look back up" and can somehow see the front door bulging and the chaos outside through the windows, which obviously is impossible when you look up through a trapdoor, not to mention that previously it was said this entry is behind the bar counter, surely blocking the sight. Kael lights an oily rag which somehow becomes a torch. There's more generic things, like somehow these Eldertides being these mythical things no one has ever seen, yet they seem to be pretty common occurrences? The dimensions of the cellar are completely unclear, at first it seems to be very small but yet they move around it quite a bit. There's other issues, like people using the same words as the narrator ("the ooze"), like they listen to him. The inkeeper suddenly calling Kael by his name like they already know each other.
Anyway, I would rate it "first draft". Of course, it is unclear whether the LLM would manage to write a consistent book, but I can fully believe that it would manage. I probably wouldn't want to read it.