You can buy a bitmain device to ship in two months or an innosilicon one to ship immediately.  So right this minute, bitmain is losing sales to a competitor.  If they have parts to build and ship miners now, then they probably would prefer to be doing so.  Very likely they still have time to get parts and build miners in time for the promised batch two sales no matter what, so not shipping anything now when they could be costs them money and supports their competitor.

The flip side of course is that flooding the market like they did with the D3s would seriously piss off the people who bought the Z9s.  But then again, pissing off their customers has never seemed like a concern for them.