While that is the impression that the article is trying to foster (perhaps as a precursor to opening the system to other retailers as Amazon has started to do with Amazon Payments), that is not what it says. It does not compare number of transactions, nor dollar volume of transactions, just how much people who already have the system, use it. Even so, while we know that use of Walmart pay among registered users has increased, without knowing what percentage of their customers have registered for it, nor what percentage of their customers can register for it (what percentage have eligible devices), nor what the dollar value of their purchases is, nor how many people who shop there use it as their primary (or frequent) retailer, we have no real way to understand the relative growth and/or value of these systems.
Fortunately, you are not running Walmart. They are very much worried about online shopping in general and Amazon in particular. That is why they have begun their pick up discount for online purchases. Having just used the system, I think they have a long way to go. When I did not receive the "order is ready" email after waiting a few hours, I contacted their support person and was told that it could take up to 48 hours. Almost every other retailer I have used that offers this (Best Buy, Home Depot, Lowes, and Staples as recent examples) have a one hour guarantee at worst. Second, the pickup location was at the far back of the store. While this benefits them in two ways (increases the chance of impulse purchases and puts it near their back stock area), it makes it much less convenient for the customer. In LA, for many purchases, I could have had it delivered faster than waiting for the order to be ready at Walmart. That is a big problem for them.
I have no idea what percentage of their retail customers also use them as their primary online retailer, but I would bet that at best they split that business with Amazon. For many younger consumers, online is the first choice and Walmart is not most people's default choice.