Yeah, you got it right except it wasn't really a surprise attack. I thought it was because he didn't understand the attack itself or it didn't fit certain parameters and that's why he goes on to talk about how clever it was after dodging. Apparently, he automatically sorts out targets by amount of cursed energy, mass, size, speed, and shape by 2007 but poisonous things were still too hard. If that's the case, you couldn't really apply it to Purple since it's his own attack and he can still use it manually.