It literally says on the homepage/the page you linked, that pixi is just a frontend for uv in the background to interface with PyPI and the project TOML files.
that isn't quite true, it defaults to conda packages (and so supports non-pypi things, its main advantage), I believe only when you are mixing these with pypi packages does it then also use the same resolution library backend as uv (and indeed directly at the rust level) https://pixi.prefix.dev/latest/concepts/conda_pypi/