All mechanics are dissociated from the narrative. That's what makes it a mechanic
I know where you are coming from, but I cannot agree with this as a blanket statement. It seems to me like saying that “sentences” and “grammar” are dissociated from traditional written narrative, which is clearly not the case. For roleplaying, I’d say that mechanics are the way you construct narrative, much the way that using grammatical rules and constructs are the way you construct written narrative. Rather than
dissociated, it seems they are fundamentally
associated.
My guess is that we are more likely to agree on a modified version of your thought, augmented by this comment:
That one key part (not the only part) of immersion is when you've mastered the rules to a sufficient degree that they get out of the way.
If a mechanic is
intrusive — in the sense that you have to consciously think about it as a mechanic rather than simply consider its effects — then I’d agree that it breaks the narrative flow and takes you out of that feel of being in a story, in much the same way that a sentence like “James had had a pleasant trip“ does. It’a mechanically correct statement, but not one that looks natural to me, so when I read it, I stop thinking about James and his trip and start thinking about the rules.
In much the same way, when i attack an enemy in an F20 game, if I roll d20 add my bonus and ask the GM if a 22 hits, I’m not thinking mechanics at all; I’m involved in the narrative, wondering if the next part of the story is me triumphantly landing a blow on the enemy, or dismayed by his strong defense. But if the enemy beats me in initiate and attacks me (PF2 rules) and I try to react to that, it takes me out of the narrative when the GM reminds me that since my turn has not yet started, I don’t have a reaction to spend — not that it’s a bad rule, just one that is not yet internalized by me. I stop thinking about the narrative and start thinking about the rules.
Another example: if I were to play a mechnic-free game in French, I could do it (badly) but it would not be possible for me to be immersed in it, because at every point I’d be thinking about the mechanics of speaking French; I have not internalized enough French “mechanics” for them to get out of the way. For me, the rules of roleplaying games are just one set of mechanics which are needed to play. Language, ability to roll dice and move tokens (whether physical or on a VTT), understanding social conventions — these are all “mechanics” which if you’ve internalized don’t stop your ability to feel in the narrative. I’ve been in enough situations where one or the other has been issue for someone at the table, definitely including me, and it does not seem to me that there is much difference in terms of interrupting narrative flow between trying to remember how to construct a future conditional in French, mixing up “!r 4d4+2“ with “/r 4d4+2”, or recalling which skills you can use to defend against a Provoke attack in Fate.