I agree with your last sentence but don't think you understood my point. More likely, I made it poorly.
Texans had driven all Mexican troops out of Mexican Texas. The Mexican army came to the Alamo because Texan troops were garrisoned there and the battle ensued for possession of that territory.
Santa Anna didn't march all that way to end slavery there, true, but his victory did just that, anyway, at least, temporarily.
The real question should be, from where did the capital come that financed that revolution - whose interests would have been best served by an American Texas?