So very untrue. The Mexican Constitution excused Texas from their general ban on slavery, and
did the Republic of Yucatán and the handful of other Mexican states that declared independence when Santa Anna declared himself dictator leave due to slavery? No, Texas had only a few slaves and was guaranteed them anyway. All those mexican states declared independence as Santa Anna was a depostic dictator.
This is just another of those myths pushed from the 'all of American History was slavery all the way down" crowd.