Actual replacement theory docs specifically mention interracial marriage. You can go all the way back to Charles Manson and find him saying similar things; he was a white supremacist. This is nothing new. Manson also predicted white men would be pushed to a point where they'd turn to violence to save the white race....
All that is true, but that Texas guy in question stated that it is about Hispanics, and he actually brought up Hispanics in interracial marriages in his writing. He probably is racist against everyone who is not white, but his writing and the actions in question are about Hispanics. His actions are egregious either way. Bringing up other people, though, really gives the vibe that you are aware of Hispanics supporting white supremacy and wanting people to ignore that, though.