What an utterly delightful oneshot. Kumo Suzume did a great job of giving both Iyo and Subaru strong personalities in a limited number of pages, as well as demonstrating how both of them view stargazing -- and, eventually, each other -- as a haven from the stresses and isolation of everyday life.
Weird that Subaru assumed that Iyo would stop talking to her if she found out she was girl. I would say she would be more assured and happy to have another female friend to share her hobby.
I think there were a couple reasons for this. First, Subaru correctly guessed that Iyo thought she was a man and was likely worried that Iyo would feel lied to or deceived upon learning the truth (I wouldn't be surprised if this happened to her before). Second, Subaru was flirting with Iyo nonstop from the very beginning, and might've thought Iyo would feel disgusted if she learned it was a girl flirting with her. That's why when Iyo reacted well, Subaru was able to work up the courage to ask her out on a date.