With respect, that seems most likely to be serendipidy rather than selection or carving and as such can't really be cited as "evidence" in support of what you're claiming.
It happened - we observed and photographed it and did the maths.
Whereas to me, Midday marked by a point (or notch or lightbox arrangement) implies an intention to mark midday on a particular day rather than every day.
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!
But I'd go further in my "what's most practical is most likely" musings:
I'm thinking a shaft of light shining through a gap wouldn't be my timepiece of choice as if the gap was, say, 5 suns wide, the light shaft doesn't move very well and ought to be much narrower and it would be a real headache to fashion the gap so it worked every day. So shadows are best for marking the time year round - at midday the sun travels about one solar diameter per 2.5 minutes and the shadow reflects that rather sensitively all year round.[/quote]
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!