The thing is that if you're actually teaching C++ (which I have done), there's still the C parts that you actually have to teach as well that are common between both. So, if you know C, you don't need to learn those over again. I actually took a course at one job on C++ for C programmers back in the 90s and it was very well done for the time.

True, modern C++ is quite different. But, there's nothing from preventing a C programmer from starting as a "bad" C++ programmer and learning new techniques as they go.