That's not central planning, that's just regular planning.

Both Walmart's supply of resources and other producers of its goods (or near-identical substitute goods) are outside the control of its planners, forcing them to spend most of their energy dealing with the vagaries of the market instead of just making their goods, which is what socialist planning was supposed to avoid.

Adding planners doesn't give you more (central planning) it gives you (less central) planning

The point is that companies like Walmart operate internally just like Soviet Union. The USSR was also constrained by a global capitalist market system and traded extensively with other countries, including the U.S.

Further, Walmart has an enormous influence, and often outright economical control over many of its suppliers, to the point that it can make or break them. It's notorious for playing hardball in their negotiations with suppliers.

Even further, there's such a thing as Walmart's brands, which place even further control over white label suppliers, and sometimes take over production on their own: https://en.wikipedia.org/wiki/List_of_Walmart_brands

Even further, Walmart owns its trucking fleet and employs truck drivers directly.