Given that Walmart operates a marketplace like Amazon, there's a good chance that some of the items were not sold by Walmart directly and were instead sold by third party sellers at extortionate prices.
I don't really run into marketplace items when I use Walmart for groceries unless I seek them out, as they are shipped the same day from the store.
Groceries in the physical store are all Walmart
This actually isn’t true. Some shelves are simply rented out to vendors.
A vast majority are not.
Right but the OP wasn't clear about what "cart" exactly entailed. You could place grocery orders online and have marketplace vendors' items in the same cart. The phrase "4-year old saved cart" also suggests it's online rather than in-store, because it's trivial to save carts online but non-trivial to do so in-store. Moreover none of the subcategories for food in the CPI is anywhere near 2x, so I suspect something else at play than just "prices going up"