I thought serverless environments are containers and thus don't need to boot?

nowadays serverless environments are microvms. that is, a standard machine type that is explicitly virtual (so device drivers have no wait time for hw to init). however, they don't either need userspace to boot because the function/application/whatever is likely init itself or not far off.

so they need to boot, but only the kernel needs to boot.

They still have a userspace to bring up.

And don't need systemd for that.