That's a tenable reading of slots, I agree. Part of the issue in D&D particularly is that in most versions there is no real provision for losing slots as a consequences of a failed action, or of a failed save to avoid hardship. So they become weirdly silo-ed like you say. In 4e, by way of contrast, losing access to limited-use powers is a fairly typical condition/consequence, and so they become less "detached" from the imagined world, and so less "meta" on your account.
The characterisation of "meta" that I know best is
this one:
compare the following: (1) an in-game essence or metaphysical effect called "Karma," which represents the character's moral status in that game-universe according to (e.g.) a god or principle in that game-world; (2) a score on the sheet which has literally nothing to do with the character's in-game identity, also called "Karma," recognized and applied by the real people with no in-game entity used to justify it. In both systems, Karma is a point-score which goes up and down, and which can be brought into play as, say, a bonus to one's dice roll. But I'd say that #1 is not metagame at all, and #2 is wholly metagame.
Mechanically, how do they differ? One thing to consider is how the score goes up and down - by player-use, or by in-game effects? Another is whether the score is integrated with the reward/improvement system - does spending a Karma reduce one's bank of improvement points? In fact, is Karma a spent resource at all? Still another issue is whether in-game effects must be in place, or inserted into place, to justify its use. No one of these indicators is hard-and-fast, however; one must consider them all at once . . . At this point I tend to think that the main issue, basically, is who is considered to "spend" them - character or player.
If we think about 4e D&D martial limited-use powers, these are "spent" by player-use, and there is no need for any game effects to be in place to justify their use - this pushes in the direction of "meta". But they are "recharged" by an in-game consideration that must be in place, namely, a short rest - and this pushes in favour of not-"meta". Who spends them? Well, the character can talk about performing the particular exploit - eg
I'm going to attack them all with a Sweeping Blow - but part of many manoeuvres is that the NPCs/creatures are positioned/oriented so as to create the opening/opportunity that makes the exploit possible to perform. And it's not the character who is doing
that.
I don't think it's a coincidence that these mechanics sit on the borderline of "meta"/non-"meta". That's a feature of some key D&D mechanics going back to hit points and saving throws.
I don't know the details of Force Points to apply the same analysis to them.