Yes. It is relative file names and common-parent-directory is nil.

On 19/12/2022 07:12, Shuguang Sun via Bug reports for GNU Emacs, the
Swiss army knife of text editors wrote:
> Finaly I found it is due the the zero length of
> `common-parent-directory` in `project--read-file-cpd-relative` as I
> using `fd` to provide the list of dirs which does not include the common
> project root in the returnd dirs.

Does that mean that it returns relative file names?