Right. In-universe one way, chronologically goes the other way.
But it seems the NX as used for Excelsior is akin to current day military usage. XB-70 Valkyrie bomber is experimental, and in service it'd become B-70. And the Raptor was XF-22 experimental then YF-22 prototype then F-22 in service.
So NX-2000 would become NCC-2000 in service, as seen.
This fits with historical precedent and makes sense.
But if all that is untrue, NX did NOT indicate anything about an experimental vessel, as you suggest. Well why wouldn't it?
And if so, what's the meaning of NX as used on Enterprise?
NX class? Random letters-class?