And you miss the point of that clause completely. A state can split up with two conditions: 1) the consent of the state legislature and 2) the consent of Congress. When Congress signed that article admitting Texas as a state which contained that clause then condition 2 was met. That leaves only condition 1 to needing to be met. Texas is equal to all other states and is treated the same as all other states in needing that approval if they want to split up. But Texas already has that approval -- the other 49 don't.
Well, no, the treaty says that “New states, of convenient size, not exceeding four in number, in addition to said state of Texas, and having sufficient population, may hereafter, by the consent of said state, be formed out of the territory thereof, which shall be entitled to admission under the provisions of the federal constitution.” Said states can be created only with consent of Congress when the split occurs. A blanket pre-approval is unconstitutional.