Yes, the monopoly argument depends on believing that somehow, even though there were these absurdly high profit margins from simply carrying a different good, no grocer other than WF or WO will ever recognize that for eternity, because, over the short time intervals that one study looked at, they never switched to carrying the goods, despite it being trivial for them to add or remove product lines for their stores, and there definitely not being monopoly power responsive for this choice further upstream.

It's just not a good argument, nor does it refute that this "counterfactual competition" does, normally, in practice, keep vendors from charging excess profit margins; nor does it contradict the long history of sellers entering abandoned markets thinking they'll be the only one, only to face competition shortly thereafter.