I don't know why it says that, it's explicitly not: https://github.com/containers/bubblewrap/blob/34a8c8bc870783...

Distros without user namespaces will probably still ship the SUID version though.

You can toggle between the SUID and unprivileged user namespaces implementation at compile time. I think most distros are shipping the unpriv'd userns configuration, and if they aren't they should be.