The lack of positioning holes does not mean that a grip can't be attached. There have been grips in the past that didn't rely on positioning holes.
Other than possibly Rebels (I haven't looked at them in well over a decade), not since before at least the 20D for Canon x0D and xD bodies.
20D-90D all have at least one guide hole in the body and at least one guide pin on the grip.
5D-5D Mark IV all have a guide hole in the body and guide pin on the grip.
6D and 6D Mark II both have a guide hole in the body and a guide pin on the grip.
7D and 7D Mark II both have a guide hole in the body and a guide pin on the grip.
The 1-Series have no guide holes, but the reason for that should be quite obvious.
They've all got guide holes.
I wish you were correct, but I don't think you are.