Yes, Tejanos borrowed a lot from German music.
Texas is full of Germans (I'm one of the only people I know who is of German descent whose family is NOT "Texan"). Until recently, there were people who grew up here speaking *only* German. My mother met a 50-ish woman recently who spoke with a heavy German accent; her family has lived in Texas since the 1840's but she didn't really learn English until she came to Houston in the late 1960's.
Since EVERYBODY seems to have settled in Texas at some point, I'm sure the Germans weren't the only polka-playing people. Texas also had a comparatively large Wendish (Sorbian) population at one point--they came here seeking religious and cultural freedom. I don't know if they traditionally played polkas but they did come from Germany.