Yes there were receivers on the field but some of the passes did not even clear the LOS and there was not even a receiver on screen when he dumped it in the dirt.There were receivers around.
My question is why doesn't everyone do this? The likelihood of the balled getting picked off when you throw it in the dirt is extremely low and you will save 5-10 yards by avoiding the sack. What is the worst that can happen? If they call it you should be close to where you would have been if you just ate the dirt and didn't Brady it into the ground.