If you are the secondary person on someone else's Walmart+ membership, that person can see information about how you use it, such as how much you saved in each benefit category. The examples given are grouped totals rather than a list of your individual purchases, and this only applies if you choose to join a shared membership. Still, Walmart says "for example," so the list is not necessarily complete — other membership-related details could be visible too. This is a disclosure to another person, not a sale of your data to outside companies.
“certain personal information associated with the membership may be visible by the primary member, for example aggregate savings by benefit category (like Store Perks) or if the secondary member accepts or terminates the benefits-sharing arrangement”
What you can do — If you don't want the main account holder seeing your Walmart+ savings activity, don't accept a shared membership — or end the benefits-sharing arrangement in your Walmart account settings and get your own membership.