You're right, you can't. You need some additional stuff. Plants takes both CO2 from the air but they also need for instance nitrogen from the ground. Now, of course 80% of air is nitrogen but it's bound up in an extremely stable state that plants can't really use. It takes too much energy to break the bond. If we could find some way to extract nitrogen cheaply from air though, it would be a real game changer.
Well plants do other stuff than only produce food, so I guess the idea is that it might be possible to produce food more efficiently than a plant does, which isn't that far-fetched.
So it's not "don't eat plants" as much as "look, we made this bacteria that can produce a vegetable-soup with slightly more calories per energy input than a carrot".