This is 1 dimensional, and we cannot see what he is seeing.
1) prone is best if possible.
2) sitting is second best if possible
3) kneeling is next best if possible
4) standing unsupported is worst case, unless we include 1 handed/1armed shooting.
Bone on Bone does not move, muscle is flexible and in the best case scenario does move. No offense to your teachers. Build your platform out of wood not rubber, even if it is good rubber.
Why would you not go prone? best defense, you cannot see over something, same for the next 2, and I have shot in courses like this. Standing supported is at least as good as kneeling.
In the picture we cannot see the whole scene, He might be a big dumbass, with a marginal rifle, or he might be using the best cover that he has. He is in uniform and that's what he was issued and someone told him to take the shot from there.
The surrounding fauna is forest, but I don't see really high grass, prone could easily be out, it looks to me like he is sitting on something, not kneeling, but he is slung against a tree, against the stock not the barrel, in kneeling the grip arm is out, not in, it looks like he has it supported. There was a position called " rice paddy prone" that looked like this, but the grip arm is definitely supported.
Look it over again and use the pictures I have tried to paint.