Sky lanterns require an open and continuous flame, which is considerably more troublesome to combine with cargo of any sort. Early manned hot-air balloons were filled by gas generated on the ground, which requires an understanding that it is the gas which generates lift through buoyancy.

Concept behind hot air balloon understood.

I’m not well-read on the history of Chinese scientific thought, but it could very easily fit into what I said in the comment: "If you lack the concepts behind the invention, the only way to discover it is by accident" That’s how most things were discovered before early modernity.

Knowing how to make something, especially something discovered by accident or passed on by practice rather than taught as an application of theory, is different than knowing why it works, and understanding why something works opens up a much broader range of applications and elaborations.