As others have pointed out, the Z9 batch 1 isn't sold out yet bitmain releasing batch 2 seems like a shrewd move.
I'm quite confident Bitmain has thought this out carefully and somehow will be making bank. My guess is August-September is a date they would have happily earned enough profits from their own private mining and are then willing to let go of the equipment to lessen the risks. Or they could know something else we don't know.
Or they are just wating to see if other equihash coins will fork by then because most of them are waiting for zcash sappling udpate that should happen in june,so potential fork to fight of asics might happen in august-september period-similar situation to cryptoknight