Help with follow up email after interview

So I had an interview for an internship yesterday with the HR director but it was super quick and ended in around 15 minutes because she said she was very busy. I asked if I could follow up with her (send her a thank you email) but she told me to follow up with the HR assistant instead.

How would this work since the HR assistant didn't interview me? We also did not touch on any important points during the interview so I'm not sure what I would even talk about especially if I am not even addressing the letter to the HR director herself. I've never encountered anything like this before as I've always followed up with the interviewer. Does anyone have any suggestions?

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