What Co-witness means is that you are able to see your iron sights through the optic. The set up you are looking at accomplishes this task.
When you use the Aimpoint, you dont want to look through your rear sight, into the aimpoint, at your front sight. Doing this defeats much of the purpose and advantage the Aimpoint offers.
Look just ABOVE you iron sights, past the Aimpoint, at your target. The dot reticle in the Aimpoint will appear on your target. This is what maked the Aimpoint so fast. No sight allignment/sight picture. Just shoulder the rifle, look at your target, and trigger press. VERY fast VERY easy.
Should the aimpont ever go out on you (fat chance!) just lower your head a fraction of an inch, and you can use your irons right through the Aimpoint. Fantastic.