The article would apply to states remaining in the Union. There was nothing in the Constitution prohibiting states from seceding, and the understandings of Virginia, New York, and Rhode Island at the time of ratification confirm that. Once a state seceded, constitutional restrictions no longer applied to them.
The rules of Great Britain no longer apply to us -- we seceded from them. Mexican rules no longer apply to Texas (yet) -- Texas seceded, and the US accepted that secession.
Texas seceded from Mexico, that is.
And nothing in it implying that they could secede unilaterally.
The rules of Great Britain no longer apply to us -- we seceded from them.
We rebelled from them. We fought for 8 years before we could say we were free from the rules of Great Britain. Our actions in 1775 were no more legal than the Southern actions in 1861.
Mexican rules no longer apply to Texas (yet)
I'd keep checking on that if I were you, gringo.
Texas seceded, and the US accepted that secession.
Again, Texas rebelled and fought a war and achieved its independence only when they won their rebellion and Santa Ana acknowledged it.