the important aspect here is not just the notch, but the relationship of the subtly curved capstone edge and the Sun's changing zenith position in relation to it. If this allows the shadow to fall across the notch, at exactly noon, all year round, I would say that is even more evidence of intent!

Don't forget you have to take into account the angle of the capturing face in relation to that movement, so yes, it would indeed be a real headache (accomplishment?) to position and work the stones so that it worked every day. That's what makes it so amazing - they did, and it still works!