I believe the problem here is that systemd can be introduced to a "non default" Debian system via package requirements, whereas they hope to prevent that in Devuan.
If I understand Devuian correctly, they will port releases of core software off of systemd and back to init, giving you the option of still using core software packages without systemd.
> Devuan will derive its own installer and package repositories from Debian, modifying them where necessary, with the first goal of removing systemd
Debian is committed to support sysvinit for the next release at least. I really doubt anyone in Debian will add dependencies on systemd "just because": if they do so they may have compelling reasons (eg. ConsoleKit is unmaintained and broken) and the correct reaction would be to fix the issue (eg. like the ConsoleKit2 people are trying to do) rather than forking Debian just to ignore the issue.
Not that forking Debian is bad and people should not do it for whatever irrational reason if they want: it's just that in this case there's no rational reason the same task couldn't have been carried over in Debian itself other than the fact that in your own fork you may accept lower quality contributions more easily.