Where does it fetch the C packages from? I always thought PDM was a _Python_ package manager, so the only source is PyPI or another index.

PDM has plugins, such as being able to invoke conda commands: https://github.com/pdm-project/awesome-pdm

Otherwise I don't know what they're talking about, it is indeed a Python package manager.

I think it's more about tool X vs Y, but about ecosystems and packaging approaches; in other words Python packaging (which has tools like pip, uv or poetry) vs conda packaging (which has tools like conda itself, mamba or pixi). https://pypackaging-native.github.io/ is an excellent starting point to learn about the limitations on Python packaging for native dependencies and compiled extensions.