Flooding doesnt kill immaterial creatures because...?
I'm pretty sure flooding kills immaterial creatures. Or was that sarcasm?
Consistency note: this might be a completely coicindental thing, but cards like eclipse, flooding, fire shield might effect quinted creatures because they are in need of a buff. Flooding after 1 (or is it 2) buffs is still a very un-used card, it needs all the boost it can get. I remember firestorm being a very, very widely used card, almost a staple along with your otyugh/ffq/druid, which is why it might fail vs quinted creatures.
As I stated, these might be inconsistencies, on-purpose, or an inconsistency allowed to stand on purpose.