Apparently Costco plans a fully separate online order picking area in one of their new massive warehouses. With that they won't have the uncertainty of shrinkage (Costco has a serious problem with theft), and of course the amount of inventory that is moving around the store in people's buggies.
I don't think there is any intention to it, but it's just a legacy remnant of being a very successful B&M company where they would periodically do an inventory and figure out where they actually stand.