For me it would really depend on how this came up. If it came up in a natural way it's a reasonable explanation.
More generally - I'm not convinced that very young kids like that are best served by a lot of historical information of that kind, in general, where it isn't immediately necessary. I think a lot of parents and teachers think they understand, but often their understanding is quite flawed. They have little sense at that age of timelines (I've spoken to a lot of little kids who, learning about ancient Egypt, think that people are stll over there making mummies and such.) They don't always really understand abstract concepts like race in a way that's sufficiently nuanced. Sometimes, unbeknownst to adults, they can really get the wrong end of the stick.
I tend to think at that age kids are best served my learning that it's important to respect all people, whatever their apparent differences, and that we are all human, and more practical kinds of differences they see in the kids and families around them.
Which is to say, I wouldn't be inclined to have a discussion in a class of six year olds about political models, racist or otherwise. But - it could have come up for another reason, something the kids saw on TV, a question that was asked. That's fine IMO.