One might think WRATH is the most stupid demon god, but they would be wrong. The stupidity of demons with the sin of wrath is because of their inability to be more intelligent. They don't have a choice but to be stupid.
Demons with the sin of pride, on the other hand, are intentionally stupid. Demons of pride can be smarter and are very intelligent, but their sin demands that they do stupid things for pride. This way, with the right words, they can be the easiest to anger.
Demons of wrath, on the other hand, get angry quicker and easier and can be angry for much longer. It is that anger that blinds them. But anger doesn't blind a demon of pride. It certainly hasn't blinded the demon god of pride. It was able to notice that something was odd about Aeternus immediately.