> V. Solution
> Upgrade your vulnerable system to a supported FreeBSD stable or release / security branch (releng) dated after the correction date, and reboot the system.
Not everyone can just freebsd-update and reboot, so yes, "Oh dear." is a good response to this.
Why can't they? Upgrading and rebooting is kinda the standard response for most security issues. So I would expect something like Ansible's playbooks for this exact scenario. You might also have it setup as a staggered rollout.
What prevents it?
Anyone relying on a 30+ year old monolith kernel written in C to not have some exploitable LPEs lurking should stay in basket weaving and out of sysadmin.