Looks like they haven't actually published the source code, despite both GitHub and PyPI claiming the project is apache licensed. If you install the package all you get in the wheel is precompiled cython libraries and the absolute minimum python code needed to import them.
Given the broader state of the Python ecosystem, that is a red flag for me. Why would you add an OSS license but withhold the sourcecode?
The only reason I can think of is to "trick" people into downloading and betting that no-one actually checks the source, then hiding something in the .so files that are shipped as wheels. Tread carefully.
Indeed, this is a major blocker. I am very wary of installing it and would never ship this to production.