The best answer is: They were US citizens.
The Treaty of Guadalupe/Hidalgo, signed in 1848, ended the Mexican-American War, and as a result, Mexico ceded a significant portion of its territory to the United States, including Texas. This meant that Anglo settlers in Texas became US citizens, as Texas was now part of the United States. The treaty did not explicitly address voting rights or give settlers the choice of being US or Mexican citizens, but it did establish their legal status as American citizens. The treaty also provided certain protections for the property and rights of Mexican citizens residing in the ceded territory.