It has become a catch-all for many Texas and Oklahoma-based bands, especially nationally (it's less divisive than the term "Texas Music," or "Texas Country"), those in the know know that there is a line.
While she's not a Texan, Morgan Wade has become of those artists who get a free pass into the Texas / Red Dirt scene. And don't ask me where to find one, I have no clue... Maybe ask BJ Barham or Adam Hood, perhaps they can point you in the right direction...