I don't think a $20 burger and drink is a simple, common social expense for everyone. Certainly not on a frequent basis.
If you're curious how I handle those friendships: I invite them to dinner at my house. Much as discussed in the article.
If most people eat lunch out once a week for $20 - that's about $1000/yr. That's trivial for most people if it's part of their social/entertainment expenses (eg even making $30-40k), but it's okay that it might not be trivial for all people.
Obviously trivial expenses are the first to be optimized away when a budget crunch occurs, because they are also trivial to remove/stop. It's hard to delay rent or a car payment withour real consequences, but there's no real consequence to not eating out lunch anymore.
Are you saying $20 for a burger and drink is trivial unless one is "dirt poor"? That doesn't match my estimates of the distribution of wealth in the US, and certainly not the world.