Being summoned by a civ with only one city would not cause the Infernals to be immediately defeated - they don't take over a city from their summoning civ, and until they found a city they don't need a city to stay in play.
If Os-Gabella was at war it's possible that Hyborem came in right on that enemy's cultural border, founded his city, got attacked immediately (since he inherited that war), and lost.
Hyborem does come into the game in really awkward places. I remember in one game he arrived just on the border of my recently moved capital when it was at 498 culture. He never seems to move to a better city site either.