That is just your opinion.
There's nothing in the episode or the context to liken the apparent callsigns to any registry scheme we have ever heard of. If they do represent some completely novel scheme, then we can ignore them all the more confidently, as this scheme doesn't overlap with the ones on screen.
It does not prove in any way, that the "X" stands for eXploration rather than eXperiment.
Exploration is specific, experimentation is generic. There is only one exploration project in Starfleet, but all
its projects supposedly start out experimental. The existence of an "NeXperimental Alpha" so late in the game thus doesn't make sense, but the existence of "NeXploration Alpha" does.
Or, more generally, NX cannot be something that would have been applied on any Starfleet vessel or project before Henry Archer's supership, otherwise NX-Alpha would have been "used up" already.
So there's nothing to prove that X is for eXploration, but plenty to prove it doesn't stand for eXperimentation - and out of the options remaining, eXploration makes immense sense.