Its production, not shipment.
15M produced by 03 2021 is probably like 12.5-13.5M shipped, which is definitely aggressive but not crazy considering where demand will be for this in the launch frame.
Think 9-10M shipped for Nov/Dec and ~3M shipped for Jan-Mar.
Now its slashed to 11M produced, so maybe 9-10M shipped. 7-8M in Nov/Dec and ~2M Jan-Mar. PS4 with a staggered launch did 7M in the same frame, so this is still a massive 40% increase over that.