There are so many layers to this gag.
Firstly, the fact that such a childish thing is being treated as serious and emblematic of a real desire to be the best.
Next, that Anna, on top of saying it completely deadpan, goes so far as to actually pull her own twintail to enforce the point. Almost as if to say, "Look, this was your goal. Don't you want to do what I'm doing now?"
Thirdly, that she actually quotes what Emo said, rather than just paraphrasing, complete with くるくる and びょんびょん instead of just 引っ張る.
Fourthly, Anna did in fact overhear Emo claiming she was going to do this, but she quotes the first time Emo said this, in episode 5, when Anna wasn't even there, instead of when she said it again in episode 9 and Anna did overhear it and questioned her about it.
Lastly(?), Emo doesn't seem the least embarrassed about being reminded she said it, even though she's not in a heated argument.
