Unlikely, probably just because C++ style comments aren't valid in the C89 standard. I assume Visual Studio, like gcc, defaults to compiling with "c89+", their own flavor of the c89 standards with sane extensions like C++ comments and not needing to declare variables before all statements. My C class forced us to always compile gcc with "-Wall -Wextra -pedantic -ansi", and I forget the "cl" and Borland arguments. (Fortunately Makefiles were usually provided.)
I mean, if this is a reason not to use C++ style comments, then it seems like a very weak one of many reasons not to use C++, I don't think that was intended.
Oh right, that would be it: this was a mechatronic engineering course and when we first learned C they taught it in Visual Studio. It sounds like your C class was better :) I wish they'd taught us initially to do the compiling/linking etc. ourselves. I guess they wanted to just teach coding and leave the rest as an exercise for us to learn independently (the irony of course being that learning all that other stuff seems to me to be vastly more complex and involved, and in need of formal educational structure, than writing code, but anyway ... )
So I guess most of the C compilers for the target microcontrollers we were working with in later courses would have used the C89 standard
I mean, if this is a reason not to use C++ style comments, then it seems like a very weak one of many reasons not to use C++, I don't think that was intended.