> What part of that is undefined? When you move the pointer, the original becomes null..

Nope, the argument is "valid but unspecified" not necessarily null. And http://en.cppreference.com/w/cpp/utility/move specifically has this as an example of UB:

    std::vector<std::string> v;
    std::string str = "example";
    v.push_back(std::move(str)); // str is now valid but unspecified
    str[0]; // undefined behavior: operator[](size_t n) has a precondition size() > n

There's an other fun UB because std::move doesn't check for aliasing, so self-assignment:

    v = std::move(v);

is also an UB.

And of course even if it were null., pcwalton talks about use after null. Deref'ing a null pointer (to use it) is still an UB.