I find this slightly unclear; does 'atheistic Buddhism' count as a 'revealed religion'?

Probably not. "Revealed religion" means something like "religion whose teachings are based on revelations from the divine to mankind". If you think that your religion was figured out, discovered, or intuited by a human being rather than delivered to him or her by the gods or spirits, it's not a "revealed religion".

Yes, this means Scientology is not a "revealed religion", because they think Hubbard discovered it. (Scientology is, however, a mystery religion, meaning that it has inner and outer teachings.)