I think it is. It is the kernel's job to protect itself from user-space mischief. It always has been. For example why are processes isolated from each other? Why does it guard against invalid memory/resource accesses etc? The kernel should be able to protect itself from denial of service attacks and must remain functional at all costs. A fork bomb is equally bad it is no excuse for the current failure mode.

BTW Linus agrees rate limiting is desirable here. This is the reason why I said systemd has to hold off its fire hose until the kernel can take care of it.

The kernel does quite a lot of work to prevent user-space programs from doing stupid stuff to each other, or to at least ameliorate the impact of such stupidity. One of the Unix philosophies has always been to prevent a runaway program from rendering the system inoperable.

systemd has apparently elevated itself somewhat above the typical "user space" level, but it's still not a bad idea to harden the interface between systemd and the kernel where possible.