This is a slip- up, even if they didn't think anyone would notice. The guy's not fighting air because he's a coward, he's fighting air because the engineers or w/e forgot to give him a computerized guy to fight with. It is definitely a slipup.
ok so what happened is they did they fight scene with the pirates and the navy to get the movements, then they made the navy fight air so they could put the CG skeltons in
now i dont know if there is a guy or not but if there is it probabally just a CG problem.