A simplistic economic model that overlooks hundreds of important factors may provide a basic Econ 101 understanding but it does not reflect how the world truly operates and proves nothing.
Sure it's a simple model. But the burden of proof lies with the person claiming that free trade needs to be bilateral. That's not some inherent property of it, or something immediately obvious. A basic look at it past "It's not faaiiiiiir" actually shows quite the opposite