But it does explicitly allow a part-owner to relicense their code to AGPL, and bundle it with all the other authors' GPL code without asking them for permission

But it does explicitly allow a part-owner to relicense their code to AGPL, and bundle it with all the other authors' GPL code without asking them for permission since that explicit clause already grants that permission. And from a practical standpoint the new combined thing (where part of the code is AGPL and part is GPL) has to be treated as AGPL, because if you don't follow the AGPL restrictions, then you're breaching copyright of that part-owner who did distribute "their part" only under AGPL; the GPL clause 13 allowing the combination is explicit about that "[...] the special requirements of the GNU Affero General Public License, section 13, concerning interaction through a network will apply to the combination as such."