I’ve never heard that theory before. I am curious as to your source.
From all that I’ve read and learned over the years, Stonehenge was a ceremonial site of significance and the site was used over hundreds of years. It started out as a rather simple earth ditch and barrow. The surrounding area is loaded with similar structures and burial mounds.
I recently read that if the surrounding complexes are a kin to churches, then Stonehenge is a cathedral.
As far as being a military fortification, sorry, I don’t see that. It’s too small in circumference and doesn’t really provide much cover. How would the shadows help and what would they be used for? Stonehenge soldier – “Hey! You, approaching enemy soldier, would you mind moving a bit to the left so you’ll be in line with the sun and this big rock here. Thanks so much for being a sport ole chap.”