The point is that they get to access the Western market with their dancing video app, but Westerners aren't allowed to access their market with the apps they make. That gives those Chinese companies an unfair advantage in potential market reach.
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
They make a superior dancing video app, so then engineers in silicon valley can go work on something else instead