Remember that a major writing project is a moonshot. And even at its flashiest moments, the people within the moonshot spent some time on a rocket ship at 24,000 mph and some time driving a rover at 8mph.
I am coming around to the view that 10x variance is possible in writers, but it has less to do with the ability to avoid left-tail variance ("no zero days") and more to do to the ability to capture windfalls from right-tail variance.
To be clear, I think that writers should have something like a "no zero days" standard, if it can be humanely implemented, but I believe such a thing is basically irrelevant to the breakthrough capacity of some writers to massively overproduce.