Responses
I think its another universe? Because when Barbatos (and by proxy lucifer) was ruling, they were in a kingdom where heroes would attack (basically lived apart from humans) and then got teleported to the human world
While as the demon lord from the oneshot mentions that they would incorporate themselves into human society (possession, curse, plague, etc) until they got bored and hid themselves within human society (no transporting or teleportation) so they were already in the human world
So if at the one-shot at the end where there was a timeskip and now lucifer is the demon king, what happened to albert and barbatos? Did albert die since humans dont live that long?? What happened?? Was there angst?? I NEED MORE ╥﹏╥