> Apptainer and singularity ce are quite common in HPC. While both implementations fork the old singularity project, they are not really identical.

Apptainer is not a fork of the old Singularity project: Apptainer is the original project, but the community voted to change its name. It also came under the umbrella of the Linux Foundation:

* https://apptainer.org/news/community-announcement-20211130/

Sylabs (where the original Singularity author first worked) was the one that forked off the original project.