Google gets the same availability from your feed and your product page

Pre-orders and drops that haven't opened now read the same in the Merchant Center feed and on the product page, so Google has no mismatch to reject.

A pre-order with units on order to sell is a pre-order in both places. The feed now also sends its expected date as the day it ships, which Google requires.
A pre-order without an expected date is left out of the feed until you add one. The Google Merchant Center card on the Apps page counts them.
A pre-order that sold out, and a drop that hasn't opened yet, are out of stock in both places, since nobody can order them right now.