The point, which they probably should have spelled out better, was that std::string contains a char* that aliases everything there might be a pointer to. It is often hard to prevent pointers being created to things; calling a function that takes a "T const&" argument, the compiler is typically obliged to assume that a pointer to the argument was taken and retained.

So, the "*count" in the example is a stand-in for a zillion other things you would actually have written, and that would also be affected by aliasing assumptions.