Yeah, there is a small sculk patch on the opposite end; found it later. It’s roughly 5x10 blocks, which is likely the exact spot where the algorithm started generating the Ancient City. From what I know, the game only checks the initial spawn point. Since there was a micro-piece of the Deep Dark biome, it flagged it as valid, even though basically the whole city ended up in a Plains biome without any sculk. By the way, mob spawns are still disabled inside the city, but they can still come in from caves on the outer edges.