I think you're strawmanning here a bit. Just because they saying "ready - fire" doesn't mean they'd always have a huge pause in the middle. It could just be to get a nice synchronus volley. Plus, even if someone's arms got too tired during it they could just wait until fire was called and shoot a little late. Plus, the person telling the archers when to pull and when to release could easily be an archer themselves or someone familiar with the process and not do that dramatic "pull! ... ... ... ... ... ... ... Loose!"
That said, I have no knowledge about how it actually happened. I'm just saying your arguing against the dumbest version of it.