It looks like roughly 60% of groceries are sold at Walmart in the US. And unlike phones, where you can choose Android easily, many regions have only Walmart to shop at.
That's the wrong metric. It should be obvious that the US market share of walmart on groceries, merchandise, and health don't conveniently sum to 100%, and that's not how you would present that data. You're looking at the % of WalMart's sales, not market share.
The answer is more like 25%: https://www.axios.com/2023/04/20/most-popular-grocery-stores