The story about Johnson being a KKK member doesn’t make sense. Prior to entering politics, he worked as a schoolteacher at a mostly Mexican-American school. Not the career for a white supremacist. The Klan had a lot of clout in Texas, but by the late 1920s, it was largely defunct due to anti-Klan politicians like Dan Moody. Additionally, the area Johnson represented has a large German-American population, among whom the Klan was not popular.
Johnson was so low most Klan chapters wouldn’t accept him.
That may have been the only kind of white collar job available to someone of his, uh, abilities. Also, we have no idea how he treated his students from the anachronistic perspective which is generally applied only to Republicans of yesteryear.