I see where you're coming from, but if she realized it was Naruto, I believe Kishi would've shown Sakura thinking about the whole situation and then angrily coming to the conclusion that it was, in fact, him. Sakura would NOT let Naruto get away with playing with her feelings. Even when he merely said that Salura was his girlfriend to Konohamaru, she got angry and punched him. If she gets that angry about THAT, then she would've been FURIOUS to find out that Naruto pretended to be Sasuke regardless if she resolved to be nicer to him or not. Playing with someone's feelings is a lot worse than joking about dating someone. Also, Sakura barely knows Sasuke and what his personality is like. To her, he's very confusing and hard to understand so perhaps she viewed this mood swing as just a weird thing about Saduke that she didn't understand. She has no frame of reference concerning Sasukes personality to properly analyze the way he was acting as something WAY too abnormal enough to result in her thinking of the possibility that that wasn't the real Sasuke. To her, it could've very well been just another aspect of Sasuke she didn't understand. Perhaps if she analyzed the scene in its entirety longer, she could've come to the conclusion that it was an impersonator--but she didn't.But before that she was saying some things and she even say that he was weird.
He comes and calls her annoying, you say that it would be something she would not let pass but how many times Naruto pranked her on the series.
I feel that that scene was left behind, i mean there are moments where a reason for Sakura loving Sasuke shows up and this moment is not there alas there's a short time skip before that scene and their first bell training.
I think for me it's more believable that she thought Naruto wasnt serious or just pranking her again if she let pass or not mostly because of the annoying comment and her decision to be more gentle towards Naruto.
ANd also Sasuke was totally different than the lovey dovey Sasuke that appeared on the bench scene, she would've confronted him and even she would have said this on her confession to Sasuke, and even there she said that he hated her, if she really believed that the lovey dovey was Sasuke she would not think he hates her and the other adjectives she said.
Another thing, Sasuke wasn't being all lovey dovey and overly fluffy with her. He mocked Naruto in a way, asked what she thought about Naruto, and then attempted to kiss her. That's not very fluffy or romantic to me. I wouldn't classify that as lovey dovey, but I guess it just depends on how one interprets the word.
A possibility as to why she said he hated her could be because as the series progressed, he became darker, angrier, and even more distant. As his character went in that direction, he kept breaking her heart with how he got meaner and meaner throughout the series. That progression As well as his actions during that progression kept piling up thus leading Sakur to believe that his feelings progressively developed into hate for reasons unknown to her and for reasons that she could not understand. It all comes down to her not understanding him at all thus rendering her unable to analyze him or understand his feelings and what he's thinking. She doesn't know if his dislike towards her is a result of something she did or what. She has no idea. He's an enigma to her.
To me, the Bench scene seems incomplete. I feel like it should be revisited. It's just weird to not revisit it ever again.
Sorry for late reply and typos--on my phone.