I read that they are conducting training to have an employee at each entrance door to inform and enforce the policy. I suspect that a large part of why they announced today that it will start being a requirement next week.
When I was at Universal Studios, they had two people on microphones at the entrance to each park interacting with the park visitors and discussing the mask/distancing policies. There, they had people who were very quick witted and sociable doing the talking. They made it more of a: "you are going to have a lot of fun today" -- "here are the things that the park is doing to protect you" -- "here are things that your fellow visitors should do to help protect each other", type of interaction.
I don't see Walmart getting people at every store who are able to entertain and inform at the same time. I think they are probably working hard to not have Twitter, TikTok, and Youtube filled with confrontations on Monday. However, there will be some confrontations on Monday. There are probably some people who have marked Monday as a day to "fight the good fight" for civil rights and make a Youtube video of fighting with the Walmart greeter.