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Not sure about Fortran - but C for sure, yes.

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.