[dfox]

The quote seems to be about the compiler/environment being permitted to offer GC, ie. permits you to write C++ implementation on top of some GC'd runtime. It does not say anything about you as a language user having enough introspection capabilities to write an GC. As a side note it seems that one can (ab)use smart pointers enough to build essentially working tracing GC on top of that, I've seen that done and well, the performance is horrible, obviously.