Speaking as a foodie who recently discovered his heritage is so Texan he can trace his lineage to The Old Three Hundred and whose family was given title to what is now Brazos State park by no less than Stephen F. Austin,
So what is the food we call Tex-Mex, really?
It's a bland-palated bastardization of norteno mexican food as interpreted by a bunch of midwesterners proud of their drawls.
“My joke about Tex-Mex is it’s invented in the Rio Grande Valley, San Antonio makes it popular, and Austin takes all the credit for it.”
Right - that part of the Rio Grande Valley where the white people showed up and got their asses kicked by the Mexicans, not that part of the Rio Grande Valley where the Spaniards set up house in the 1790s.
Authentic? Inauthentic? Don't care. Tex Mex is what happens to Mexican food when it's prepared for people who hate flavor.