Those fast food items you listed are usually a part of a meal that includes fries, which add another 400-500 calories.
You were comparing their purported meal of 500 calories to a single item within a typical fast food meal.
Nobody was claiming that fries + burger don't cost more than a burger.
Apparently, the standard (non-embiggened) Big Mac meal at McDonalds is their most popular option. If you get a Diet Coke with that meal, it appears roughly comparable with a meal at Eatsa.