That could be part of the reason, but also I think insisting on chip all this time just really slows down a store that doesn't really spend on cashiering all that much. Tap to pay can shave off a few seconds on every transaction (ever been behind someone in the store who finally pulls out their wallet, finds the business end of their card, inserts it, waits 20 seconds...).
Millions of transactions a day, over thousands of stores, it can make a huge difference in time saved.
This is even worse now that tap to pay has proliferated. Customers who are not Walmart regulars (and some that are) will wave their credit card, phone, or watch over the terminal and wonder why it’s not working.
Cashier must explain: No tap. Best case is the customer shrugs and inserts card. Bad case is the customer is confused or argues about it. Horrible case is that the customer must fish through his belongings for a credit card or cash and takes minutes to realize he has neither.
I guess Walmart figured that these scenes repeating daily was not worth whatever its strategy was.