True, but the significance is that crystal clear mental image and her realization about how it made her feel. Mitsuki pictured Aya there, sad and lonely and missing Mistuki, and that made her feel something that hit her as hard as a Shounen protagonist awakening their latent superpowers for the first time lol. Presumably "I'm making her feel sad and lonely because I'm not there with her. She wants me there with her. I want to give her everything she wants for Reasons. I'm getting my dumb ass to prom asap."
Turns out Aya isn't just moping around with her phone and Mitsuki will find her completely wasted off of non-alcoholic juice.
Aya decides to have fun by herself instead of getting all depressed in a once-in-a-lifetime event. Mitsuki finds her laughing with her friends and mistakenly thinks that Aya doesn’t need her. Enter the next arc: separation.
I doubt it will go this way though with all that build-up since last chapter. But if it did go this way, what a waste. Author has no mercy.