p7 says that for do_is_equal,

For a derived class D, a typical implementation of this function will immediately return false if dynamic_cast<const D*>(&other) == nullptr.

Once LWG 3000 is resolved, none of the concrete memory resource classes in the standard will perform a dynamic_cast in their do_is_equal. Given that, the assertion in the note that performing a dynamic_cast here is "typical" seems rather questionable.