Haven’t seen the movie but I assume the battle is San Jacinto.
It was so one-sided because the Mexicans went into siesta, and their general was dallying with a local lass. The battle was over in minutes, but the killing went on for quite a while after the soldiers started trying to surrender. Texans wanted revenge for Goliad and the Alamo. (And probably for how scared they were while running away.)
Though technically they didn’t really have much right to complain about the massacre at the Alamo. The men in the mission were told they’d all be killed if they didn’t surrender and they chose to fight anyway.
At Goliad, OTOH, they surrendered on terms and were later massacred after Santa Ana overruled the local commander.
But “Remember Goliad” just doesn’t roll off the tongue like “Remember the Alamo!”
MadMen at work, even then.