There were about 20,000 Germans in Texas. The counties where they settled were the center of Unionism in Texas. Many of the Germans headed to Mexico to avoid being drafted into a war they didn’t support. In one case, confederate cavalry ran down one group of 60 heading to Mexico, massacring more than half of them, many after they’d surrendered. The oldest Civil War memorial in Texas is dedicated to those victims and is inscribed “Treue der Union.” It is one of six locations in the country where the US flag flies permanently at half staff.
I've seen a figure of 2,179 Texans who served in the Union Army (Source: Lone Star Blue and Gray by Wooster). That figure includes some 443 Hispanics and 500 Anglo Americans (Germans, Irish, etc.) recruited in the Rio Grande Valley. I've also seen somewhere that the Germans who were killed in the incident you posted about were headed to Mexico to join the Union army. Don't know if it is true or not.
The Texas German community, largely concentrated in the Hill Country, provided almost 2,000 men for the Confederacy per the following words from a plaque in New Braunfels, a German town in the central Texas Hill Country.
They proved their loyalty to their adopted country by fighting for the independence of Texas at the Alamo, Goliad, and San Jacinto, by participating in the war between the United States and Mexico, and by raising twenty one companies of nearly two thousand men for the defense of the Confederacy.
I am also reminded that Fleet Admiral Chester W Nimitz of Fredericksburg, Texas was raised in Fredericksburg by his grandfather, who served as a Texas Confederate cavalry captain. BTW, Admiral Nimitz was in charge of over 2,000,000 troops in the Pacific in WWII.