Yes, Shopify inventory will always be adjusted according to the quantity of products sold from Marketplace Order (regardless of whether import order is enabled / disabled), as long as the products are already synced.
However, if products are unsynced, then Shopify inventory will not be adjusted, because we do not have the mapping yet between Shopify and Marketplace products. So please ensure that you have synced all the products that you want to keep their inventory in-sync.