No it's not - the compiler for MSL is of course C++ because it's LLVM but the runtime is absolutely written in objc (there weren't even C++ bindings until recently).
What you are talking about are C++ wrappers around Metal Objective-C API. Yes, it is weird as they are going C++ -> Objective-C -> C++. Why not go directly? Because Apple does not ship C++ systems frameworks.
The term is Objective-C++.