At least as of a few years ago, it was indeed still true.
Walmart is huge in online shopping as well. They use this position to essentially tell vendors what they can charge if they want the shelf space. If you don’t say “ok”, and they can reproduce your good, then they absolutely will if there’s enough demand to bother. This is one fantastic reason to hold a patent on your goods (if patentable).
I wonder if that is still true due to online shopping.