From a meta perspective, they would have to be considered contenders to deserve having their own rulebook and models, and being featured as a playable faction in a number of WH40k games, no?
As I mentioned, their territory is rather small and, as you alluded to, their manpower and manufacturing capabilities are much more limited compared to something like the Imperium, but as far as the story and setting is concerned, the Tau generally can't be ignored when they do make a significant appearance near one of the major factions.
The fact that the full extent of their existence has flown under the radar thus far is actually what has given them the time to advance as far as they have and, as I understand it, in some of the more recent Tau lore, certain parts of the Tau military are slowly figuring out the fact that they are a tiny fish in a big pond, and just how scary the rest of that pond is.