Because there were survivors of the Alamo battle from the Texan side. We all know the story of everyone being killed, but by "everyone" it specifically means all of the fighters, and did not include many noncombatants who were spared by Santa Ana and his men.

As for "the line" drawn in the sand, this story has no first hand accounts and is most likely false.

You can find more info on this specifically in Exploring the Alamo Legends by Wallace O. Chariton, page 195 (you can find it by searching google scholar.)

The large majority of survivors were women and children. Notable exceptions were a slave named Joe and a Mexican turncoat named Brigido Guerrero who, once realizing the battle was lost, locked himself in a cell and convinced the Mexican soldiers he was a prisoner.

In total, there were under 50 survivors, and they were allowed to go free. Each one was individually interviewed by Santa Ana, and he surely would have wanted to know every detail of what happened inside.

Alamo Defenders: A Genealogy, the People and Their Words by Bill Groneman