Iron golems and cats spawning on string

The title says it. Maybe it is not a bug and it is meant to be like that, and if that's the case sorry and i'll delete this thread as soon as posible.

 

I made an iron farm and iron golems spawn inside the villager chamber. Apparently they can spawn in bottom slabs, that's a thing apparently. I read in a few places that string disables spawn so i FLOODED the chamber with string (airblocks? nah, S T R I N G is better), and both cats and iron golems are still capable to spawn in the string. Is this meant to be like this?