OK so those processes are launched not by systemd, but by dbus itself.

There's probably a /usr/share/dbus-1/services/org.freedesktop.IBus.service file in your system and if dbus sees something that tries to talk to IBus, and IBus is not running yet, dbus will launch it for you as directed in that file. In it's own namespace unless directed otherwise.

There's an optional integration between dbus and systemd, look for SystemdService in man dbus-daemon. IBus does not set it. Perhaps it should. I don't know.

> I ran into this problem because ibus runs later than setxkbmap and undoes the keyboard settings.

that must've been pain to debug :). I can see on my system that there's a systemd user service that I could launch with `systemctl --user start org.freedesktop.IBus.session.generic.service`, maybe that would work better than on-demand via dbus in your case.