The person I replied to said that it’s really easy to change the kernels on distros, but hard to change the init system from systemd. However, most custom kernels on distros are just Linux with patches, but the core functionality and API are mostly the same. I’m pretty sure it would be easy to change the init system to a fork of systemd with some extra patches.
I don’t have any issue with other init systems, the only reason I use systemd is because NixOS was built to use it.