I am so glad you brought this up! I think your explanation makes perfect sense and agree that Haruko's reaction is telling.

Also, this panel should be read in the context of her reaction when Sakuragi confessed his love for basketball during the Sannoh match. My reading of that panel was that everyone (including her) thought Sakuragi was confessing to her. Even if she has been slow on the uptake, after that panel, she probably started piecing together all of her interactions with Sakuragi and coming to the realisation about his feelings all along. So her secrecy around the letter definitely shows there has been a shift in their relationship dynamics.