I answered a question about this the other day. Benioff is correct in the sense that there are accounts of giant piles of bodies on medieval battlefields. However, simply taking these accounts at face value is ludicrous, as medieval chroniclers are often not describing things in a factual way. Just as we don't take medieval assessment of army sizes at face value, we should also look critically at accounts describing massive piles of corpses (and fighting on top of the giant body piles!). Critical examination of sources and weighing different accounts against each other is an important part of being historian. However, if you are a hack television producer, you can just mine historical sources for stuff that sounds cool and then claim authenticity without regard for the reality behind such accounts.

I answered this question as well, with an example from Classical Greece, here. The point is that it could happen on occasion, although the circumstances had to be very particular (for instance if a fleeing force was trapped in a tight space), and u/MI13 is right to be skeptical of source accounts of bodies piled high.