The end of the article details the steps necessary for people who want to run Gnome without systemd. There is a monoculture of sorts, which is that bespoke init systems aren't tested or supported by Gnome itself, but like you already need to do to get Gnome working normally, you can still patch in support for whatever system you prefer. The article even provides a list of services and APIs that you will need to hook up to Gnome to make it work.
You don't need to reimplement logind or D-Bus, but you will need to patch your mechanisms of choice into Gnome itself. Gnome isn't planning on maintaining a second copy of common system services that exist on modern systems anymore (a copy that is based on a hack in the first place). The burden of maintenance now falls on whoever wants to provide their own alternative.
All the extra work you need to do to get alternative init systems working is work the Gnome team no longer needs to do.
You don't need to reimplement logind or D-Bus, but you will need to patch your mechanisms of choice into Gnome itself. Gnome isn't planning on maintaining a second copy of common system services that exist on modern systems anymore (a copy that is based on a hack in the first place). The burden of maintenance now falls on whoever wants to provide their own alternative.
All the extra work you need to do to get alternative init systems working is work the Gnome team no longer needs to do.