I had a survival world and came across an outpost. It was located in a mesa biome. It didn't make sense to me because the minecraft wiki states that outpost can only spawn in biomes where villages would also be able to spawn. The outpost is approximately 6 blocks away from a desert biome. It does spawn pillagers, but they spawn slower and only spawn in the desert biome. If you want to check it out yourself, the seed is -143949197830770696 and the coordinates are x = 310 and z = 225