My pack has only 6 biomes and two of them are ocean biomes which I want to avoid having cities, so I added them to the biome factors as so:
"cityBiomeFactors": [ "project_tundra:deep_frozen_ocean=0", "project_tundra:frozen_ocean=0" ],

However, cities are still starting/spawning in those biomes (my profile has city chance super low, 1.0E-4).

Forge 36.1.32
LostCities 4.0.9-beta