They meant that you cannot once a piece of code is released by GPLv2 you cannot unrelease under GPLv2. So even if every new version and contribution is required

They meant that you cannot once a piece of code is released by GPLv2 you cannot unrelease under GPLv2. So even if every new version and contribution is required to specify a GPLv3-or-later license all previous contributions and versions are still in GPLv2-or-later, which is more permissive. I think the comment was addressing a misconception around how the project as a whole is licensed and its individual contributions or past versions are licensed and the fact that GPLv2-or-later works cannot be revoked as the user can always freely chose between the GPLv2 and the "later".