This is a ridiculous assertion.
The local Costco is "at such a scale in the modern day" that it, too, is essentially a public square. It's still private property, though. If you show up in Aisle 6 trying to convert people to Mormonism, a Costco employee will ask you to leave and stop harassing their customers. Yes, the same principle applies to Twitter, Facebook, X, Truth Social and Instagram.