In the SF store where they first rolled it out you could literally see them just set up shelves and fridges upstairs and duplicate Instacart's model. Except for two advantages:
(a) Free 2 hour delivery to Prime members / absolutely no fees apart from tip.
(b) Better inventory data. This is huge. It's directly integrated with WF's logistics backend. It radically lowers the missing item / substitution problem that is the source of the worst outcomes with Instacart.
Having said that, all the other grocers are freaking out about this so there may still be a growth opportunity for Instacart.