It was made in 1978. Believe it or not, back then a lot of people used the N-word quite casually. A lot of people considered it a very bad word that one should never say, but they didn't lose their minds and go running for a safe space when they happened to hear someone say it.
No, I don't see why they would use it when there hasn't been a black person in the movie, at least not one that Wheelock would have interacted with. Even in a world where it was casually used, using it here with no correlation makes no sense.
But, as the other poster mentioned below, it does have a purpose, which previously went totally over my head.
The point here isn't that you were confused by it. It's that you were obviously distressed by it. Distressed by a slur that was used in a movie almost 50 years ago. "omg so unnecessary and pointless!" You couldn't just shake your head and go about your day, you had to come and virtue signal about it. Get fucked.
It really is significant. It shows that this clone is going to grow up in an "appropriate" environment... Maybe not just what the Nazis had in mind, but something good for fostering a little Hitler.