Why shouldn't fluidic space be accessible from the mirror unvierse? In Places of Exile it was accessible from numerous timelines of our universe, just like the STO timeline and mirror timelines are.
But you're just restating the assumption that we're trying to examine the vailidity of. Does STO really portray fluidic space in a way that's consistent with that alternate-timeline interpretation? That's the question we need to resolve before we can treat that interpretation as an axiom.
While it may be either a parallel timeline, or a divergent timeline, or simply a parallel story, it seems apparent that the creators of STO intend for it to be a continuation of "our" Star Trek timeline, whatever that may mean to each of us. And since its drawing on common events - Titan, Hobus, Shinzon, etc. - it seems possible to draw a divergence point. Whether it's the STO timeline diverging from OUR timeline, or the STO timeline continuing on its parallel track in the multiverse, yet diverging from its similar events with our timeline, seems largely irrelevant: at some point in the past (Titan, Shinzon) the STO timeline was very like our own, if not completely identical (Elachi, 8472). Somewhere around 2379-81, it began to divert. I'm curious why that is, or what may have happened (or not happened)in both timelines to cause the split.
I think the intent was for it to be a continuation of the canonical timeline, but since they needed to build a larger open-world (?) universe for the game, they drew on whatever useful material they could find in the novels and comics just so they could pad it out. It wasn't because they wanted to posit some kind of timeline divergence, it was because they needed material and this was material.
There's a long history of different extensions of a franchise drawing on each other's ideas. I've recently been listening to the old
Adventures of Superman radio series from the '40s (available on the
Internet Archive), and it's interesting to note the areas of cross-pollination with other versions of Superman. The radio series introduced the
Daily Planet, Perry White, Jimmy Olsen, and kryptonite, which were adopted by all other versions; it also established the basic opening narration later used in the Fleischer cartoons and '50s TV show. But it was different in a number of ways, too. Its Jimmy was a blond, 14-year-old copyboy rather than the redheaded and (I think) slightly older photographer of the comics. Its Superman was more secretive, at least in the first couple of years of the series, and tried to avoid revealing himself openly. And it never used comics villains like Lex Luthor, instead featuring its own recurring villains that never showed up in the comics. (There's an interesting one called the Laugher, a morbidly obese, jewel-adorned criminal genius who's constantly laughing at others' misfortune and counts Superman as the one worthy rival to his superior intellect. He's basically a mix of Luthor, the Joker, and the Kingpin. I'm still listening to his debut serial, though, so I don't know yet if he'll be recurring.) But mere weeks after the second Fleischer cartoon,
The Mechanical Monsters, was released, the radio series did a storyline with an almost identical "Mechanical Man" playing a very different role (and not used very well, since up to that point the radio show was unaccustomed to pitting Superman against such comic-book dangers, generally going more for gangsters and saboteurs and avalanches and fake tribal curses and the like).
So a branch of a franchise borrowing ideas from another branch doesn't imply an attempt to suggest a shared continuity. It's just taking advantage of shared ideas, an indirect creative collaboration of sorts. We're all playing with the same toys, and if someone else adds a new toy to the box, the rest of us may want to play with it too.
(By the way, out of curiosity, does STO use any of the characters or concepts that I introduced in the novelverse? I know they disregard my take on Species 8472, and their "Elachi" thing doesn't mesh with my take on the "Silent Enemy" aliens, but is there anything of mine they do use? Well, not "mine," since CBS owns it all, but anything or anyone I came up with?)