They have that limitation, but it doesn't manifest in quite a simple way. If the container's glibc and all other binaries require Linux 6.3, you're not running it on a 5.18 host.
FreeBSD is developed as a whole unit. It makes it easier to reason that a 13.2 userland doesn't run on a 12.4 kernel.
> If the container's glibc and all other binaries require Linux 6.3, you're not running it on a 5.18 host.
FWIW, it would take quite a long time for glibc itself to start requiring Linux 6.3 on its current targets. Since glibc 2.24 (Aug. 2016), the minimum supported kernel has been at most Linux 3.2 (Jan. 2012), on every target that has existed that long. And that's the smallest that the gap between the glibc and Linux releases has ever gotten (except for new targets). Extrapolating, we'd have to wait for glibc 2.47 (Feb. 2028) to require Linux 6.3 (Apr. 2023), if that gap were to be repeated, something which I don't find particularly likely.
That is to say, with the exception of new targets, you'll rarely see a userland that absolutely requires the latest Linux kernel, even possibly if it suggests that kernel for the best support.
It's the "all other binaries" part that will catch you. You've completely overlooked that part.
For example: Per its own README, systemd requires Linux 3.15 at absolute minimum, recommends Linux 4.15 for baseline functionality to at least work, and specifies Linux 5.7 for full BPF functionality.
FreeBSD is developed as a whole unit. It makes it easier to reason that a 13.2 userland doesn't run on a 12.4 kernel.