[locustgate]

To my knowledge the Democalus gulf is the very edge of were humans have settle, so finding a STC beyond it is very very very very E10 unlikely. Before the etherial arrival the tau were in the stone age, so reaching another planet is impossible. How ever what is kind of possible is that a GC era scout ship crashed on T'au and the etherials kept that hidden till they were able to unite the Tau. There is 1 major difference between Tau plasma and early human plasma weapons, the early humans were able to make stable man portable plasma guns, the Tau attempted to do that but was unable to so they decreased the strength.

 

[locustgate]

Its unlikely given that there's no evidence of human habitation of the core Sept worlds.

That being said the Tau's spaceship technology received a massive boost when they conveniently stumbled across a derelict ship with functional warp drives on one of the first (the first?) worlds they began to explore. It would be like us finding and reverse engineering a alien spacecraft on the moon. It's theoreticclly possible that the ship in question was a relic of pre-Imperial technology, though there's no evidence for it.
@locustgate The tau weren't quite at Stone Age tech when the etherals arrived. By the time they turned up the tau were at a blackpowder level, increased technological progress stifled by internecine fighting.

If the Tau had discovered an STC system then they'd be far more advanced. As it stands they're only at par or behind the Imperium's technological level. Were they possessing of Golden Age tech they'd be light years ahead.

The tau didn't get their warp tech from a derilect ship it was from a captured Imperial ships during the DG Crusade, (Farsight). Thanks I got when they first discovered.

 

[locustgate]

That doesn't make sense.

The Tau already had an empire, albeit a small one, at the time of the Damocles Gulf crusade. That's the whole reason there was a crusade in the first place; an upstart, expansionist xenos empire was threatening local Imperial space.

To have that empire necessitated having warp travel.

Either the Codex or the Tau's BFG fluff states it was a crashed derelict ship on a nearby planetoid that gave the tau warp tech. It logically has to be.

I can't find any mention in the Tau Empire dex of a crash landed imperial ship, before the Crusade.
The tau had near light speed engines.

Location 469(Farsight on a Kindle) :
"The earth caste's invention of the ZFR Horizon Accelerator Engine revolutionised the way the tau crossed the sea of starts. The device allow them to travel at near light speeds, thereby circumventing the worst of the Gulf's malevolent attentions"

Location 776 "O'Vesa made some astonishing dicoveries in the depths of his labs about the esoteric technologies used by the humans' fleet."