Am I right in thinking that when pex expands to a venv (if it was built in --venv mode) then the first-party source appears as a "fake dist" of sorts under

site-packages

in the venv?

Yes: https://github.com/pantsbuild/pex/blob/40bff7fa2d94b4f4158c8ff812c6af0302106ab1/pex/tools/commands/venv.py#L77-L112