Answer

First, defining plagiarism is very complicated. In the history of ideas, you will always find earlier sources for every new idea. The one who first conceptualized it and gave it a name is the owner of the idea and its copyright. After I published "The Two Wagons," I received many responses along the lines of: fine, it’s all already in Rabbi Kook / Rabbi Nachman / the Maharal / Rabbi Tzadok, etc. And they can all be right. But none of them conceptualized or named the distinction between the analytic and the synthetic, and the same is true in the world of general thought as far as I know.
Second, the reasons given by the Chinukh are just reasons offered in a general sense. They need not be taken too seriously. He is trying to put a rationale into his son’s mouth (the book was written for him).
Third, it may be that this is only an attempt to persuade a person not to steal, and not the original reason for the commandment. Many people tell their son or student: if you steal, they’ll steal from you.
Even if he means the categorical imperative (and it doesn’t seem so), there is no plagiarism here, for the reasons above.