You know BUIS stands for back-up
iron sights, right?
Yeah, the co-witness method is just a convenient way to zero your sights. They should naturally be co-witnessed anyways after both are zeroed correctly, but it is possible to zero and use the red dot sight with no form of irons on whatsoever.
I'd say the vast majority of people that have BUIS on their rifle have them for looks only, and don't bother with them at all unless there's a critical failure of their electronic optics.