It’s really logical with the hypothesis that Arcezon won but Lelia sacrificed herself (exactly like the reference to Xerath’s war in League of Legends, where Lelia had to be sealed along with the monster so Hajaan could close the gate, sealing the space). But if this hypothesis is on the right track, Part 3 would be about another nun rescuing Lelia and retelling the story with explanations for everything. Forget it, I’m going to sleep for a bit. My brain’s got issues, and I’ll probably dream about something weird
My guess is that the ending slug hints at another stage of the final boss that will be added in a future update.
In the first game, the final boss is Rabiane reaching the heart of the ritual (where reality is so warped that it looks like her safe-room) and allowing herself to be impregnated by the avatar of the evil god, so she can purify it "point blank".
It is visually extremely similar to the pregnancy purification fight in Sinisistar 2, and you can see her pregnancy progressing throughout the fight if you need several tries, with the fail state achieved when she gives birth.
Purification of the entity's foetus, this close to the heart of the ritual, unravels it, and we can see the evil god's giant avatar vanishing.
Now, in Sinisistar 2 the final fight is about purifying the Otherworlder's giant avatar, but we don't get any confirmation that it actually stopped the Otherworlder. Rabiane did NOT fight the giant avatar, she went for the ritual itself.
Which was a lone slug, in a warped room.
Which is exactly what we see after Lilia purifies the final boss.
So chances are - unless I'm completely wrong and the post credit slug is just a cheeky reference - that Lilia still needs to go through the same pregnancy ritual on steroids that Rabiane went through, to really get the good ending.