right, I have found what the problem is. The baud rate used by the flash script is too demanding for the zero. Just replace -b 460800 by -b 115200 and you should be able to refhash a firmware on the ESP!
I am going to change the script to avoid this in the future and set a safer rate for all potential hardware... it will take a bit longer on other hardware but hopefully nobody else will fall for that one again.
Sorry, I should have caught that one and test on a zero when you first reported, it just wasn't clear that an initial attempt flashing firmware was when you first ran into trouble. Well, either that or somehow your original firmware got corrupt, but that hasn't happened to me so I can't comment on whether that's a likely cause.