Today I'm torn. On the one hand, I'm pretty sure that Ernie knows perfectly well that it's Bert and he's just being a jerk. On the other hand, he's so very funny in this sketch. My favorite lines are: "With your eyes, Bert!" and "I'm going to call the Missing Bert Bureau!"
Sometimes Bert is a jerk and sometimes Ernie is a jerk. It's one of those depending-on-the-writer things. Sometimes neither of them are jerks, but I can't think of a situation in which both of them are jerks at the same time.
In this case, Bert should have just gone to his party. Although, in a way, I think Ernie did him a favor because I think he looks silly with his hair slicked down. And we all know how he hates looking silly.