To be fair, this episode's treatment of the mycelial plane and Culber's survival there was nonsense, in terms of scientific plausibility. Sheer codswallop. Pure malarkey.
I mean, we're talking about fucking spores, people. Spores do not have minds and intentions of their own. They're not even full-fledged beings as such; they're the fungal equivalent of seeds, except much smaller.
Heck, even last season on this same show, the whole concept was simply that the spores were a real-universe thing, except that their natural growth network extended into some special plane of subspace that could be used for transit. That was weird enough on its own... but the concept was definitely not that spores were an intelligent species unto themselves that treated that plane as their home dimension, much less that they possessed powers of telepathy and matter transmutation. It's frankly ludicrous.
But you know what? I honestly don't mind. Star Trek has always been on the "applied phlebotinum" tier of SF, and this isn't even in the ballpark with some of its more extreme scientific whoppers. So long as it's reasonably consistent with itself, and it adheres to valid (in-universe) principles of metaphysical naturalism and doesn't start attributing anything to actual supernatural causes, I'm perfectly fine with it. So if the (re)definition of the mycelial plane presented here provides the setting for a well-told story, and in the process brings back a worthwhile character and (perhaps) helps explain why the mycelial network is Not A Thing in later Trek, it's all to the good.