Everyone does groceries probably once a week. It would at least 30% of the population goes to walmart so its not hard to imagine 100 million weekly visitors.
Walmart stores are also convenient to a lot of people. I'm not their target demographic and don't really like grocery shopping there in general. However, one is nearby and it's pretty common that I'll pop in to pick up something even though I don't generally do a "weekly grocery shopping" there.
This isn't true by a longshot. The 300m figure is every man, woman, teenager, child, and infant in America, and you only need one person per family going for a weekly grocery trip. And that's of course ignoring the people who don't do weekly groceries. On top of THAT, the 30% number doesn't seem particularly reasonable either (though that one is at least the only remotely plausible thing you've said).