Have read Sixth Column (some time ago, and I haven't reviewed the crumbling paperback in my boxes lately), and it's plenty racist, in a very WWII Yellow-Peril-hangover way. It's so pronounced that it reads as satirically racist to any post-60s sensibility, issues blown up to comic-book breadth and, um, lack of depth, so it kind of has the opposite effect now.

I can't know, but I think it was written to appeal to the post-WWII-hangover readers of the early 50s. Heinlein always said that he was writing for a marketplace, whatever marketplace he had to.