:/ I think it needs reminding that the Damocles crusade which slammed into Tau space was a Crusade - EG- The greatest logistically feasible fighting force the Imperium can put together.
The fact that the Tau ground it to a stalemate means that the Imperium cannot 'bitch slap' the Tau away.
First, I bet that if you put something the Imperium REALLY wanted on a planet somewhere you could see something a hell of a lot bigger then the Damocles Crusade coming to get it. If you convinced some important people there was a complete STC on T'au, I guarantee the Tau would have a bigger problem on their hands. But second, you misunderstand what it means to say the Imperium could "bitchslap the Tau away". Answer me this: What would have happened to the Tau had they lost that war? Do you think they would still exist as a relevant faction? The Tau are one war of extermination away from defeat, and that is what someone means when they say the Imperium could bitchslap the Tau. The Imperium has the capacity to eradicate the Tau, even if it meant that the Imperium could only win a Pyrrhic victory and suffer on all other fronts. The Imperium couldn't eradicate, say, Chaos, or the Eldar, but the Tau could be totally wiped out.
Tau technology is nowhere as advanced as even great crusade era tech, calling them nearly dark age of technology level advanced is kinda strange.
In fairness, Tau do have some Dark-age Era tech, but only stuff like terraforming (the only terraforming the Imperium does is with Cyclonic Torpedoes). But yeah, the Tau don't have access to Dark-age era weaponry or anything supremely useful for winning in the 42nd Millenium.
Also, to show that I'm not Kot and I don't viscerally hate the Tau on principle or anything, I'm excited for when Battlefleet Gothic: Armada releases the Tau faction. I enjoy that game immensely, and I'm curious to see what they can do with them. Tau can be interesting when done right.