That is a myth. The only thing different in regards to Texas statehood is that they could, if they so desired, break themselves apart into as many as five seperate states, but they would all remain in the Union.
Sec2 of Article 1 of the Texas Constitution states:
“The faith of the people of Texas stands pledged to the preservation of a republican form of government, and, subject to this limitation only, they have at all times the inalienable right to alter, reform or abolish their government in such manner as they may think expedient.”
You are correct. But the timeless words of Thomas Jefferson from the Declaration of Independence still ring true that essentially man was not created for government benefit but the other way around (as a brief paraphrase). It is pointless to discuss whether a people or a state have the "right" to "secede" or abolish their current form of government.
If such a thing could be done by consent then by definition no tyranny would exist.
At this point I could care less what pretense we use to fight for the preservation of our cherished freedom. A fight will be waged nonetheless.