He relies on the emotional weight of the symbol to perpetuate his lies. The Alamo, although mythologized in American History, was actually a bunch of Texans, defying the lawful order of the Mexican government to either give up their slaves or leave the territory. It wasn't a heroic stand against an invading horde. It was the Mexican army trying to push out the invading horde.

All Trump knows is that "Remember the Alamo" was a bigoted battle cry depicting Mexicans as "other" and evil. What he knows is that his supporters are likely as ignorant of the actual history as he is.