Pay attention to their monologue there "Is stepping in really going to help?" "What will I do if I break everything again?" "We're just classmates in the same class after all." "Should I really risk everything during their precious youth?" It may seem presented as a back and forth between Chizuru and Arata (and what you may initially think since the only one who would treat them as youth would be Arata) but after a reread, it could and probably was actually a monologue that the both of them share.
I think there's also the fact that Chizuru dresses very maturely here, presented a lot more as an adult and office lady rather than a teenager. Guess that's also partially the reason Arata saw his senpai in her