Horse Skeleton trap are part of passive mob cap

It appears that the horses summoned by the Skeleton trap mechanic count as passive mobs, even if they look scary. It is not really an issue but when a large amount of Horse Skeleton trap are in the spawn chunk, no passive mob does spawn (only generate with the chunk, no real spawning). This break the possibility to make an efficient passive mob farm, because there is no way to disable lightnings spawn, even with a layer at maximum build height.