@joe @thephd @euneuber found the relevant bit in the video, starting at about 58:00, in response to a question about pointers:
[58:45] "[...] the same way you can't override for `int, int`, you can't override it for `int *` or `MyStruct * [...] [A pointer is] still a built-in type, even though it's a pointer to your type."
So, a compiler would issue an error for:
_Operator(<<, FILE*, const char *, fputs);
...because both operand types are built-in types...