Exactly spot on Dave!
This is one of the points that got drilled in to us during an advanced Photoshop course I took a few years ago.
Modern post-processing tools can do absolutely amazing things, but our visual system has a lifetime of experience in seeing the real world, so if something doesn't look quite right, we will notice that right away. In this shot, a scene where we see a boy walking along a beach wearing shorts suggests a summer shot on a reasonably warm day. In those conditions one would reasonably expect lots of humidity in the air which would soften the objects in the background. It doesn't so we don't quite believe what we are seeing.
Had this shot been taken in the winter, with ice and snow, then we would expect objects in the distance to be clearer, as the absolute humidity in the air would be low. We might, however, be suspicious of a boy in shorts on the beach, under those conditions.