You're right that cutting food costs is hard for someone already spending near the minimum, but the post is about the difference from eating lunch and drinking coffee prepared and possibly delivered by others. In some cities, that's probably about $20/workday, or $100/workweek, for about $5000/year. If we throw in dinner, the cost difference might be another $20/day, not limited to workdays.
Being able to cook good meals at home is a life skill worth mastering, and pays off in both personal finances and in long term health and enjoyment. People should still want to be able to do this, regardless of financial status.