It is worth emphasizing that the kanji 「彼」 is almost always read as「かれ 」/"he/him," which is what Reika used to refer to Chiaki in past chapters. The other reading of 「あれ」/"are" is written out in hiragana basically 100% of the time, very rarely in kanji. I just mentioned it 'cause I thought there might be a slim chance of ambiguity. Also Reika refers to Chiaki with "-kun" (male honorific, only gender neutral in workplace). The only other series I can think of that refers to a girl with "-kun" is Kashima from "Gekkan Shoujo Nozaki-kun," but that's part of the character's shtick.
One thing I did forget to mention that is pretty major to this chapter is that Chiaki uses 「オレ」/"ore" (male pronoun) in the flashback. Which would contribute to Reika's "perception" of Chiaki. That one is my bad and will add a TL Note A.S.A.P.
For the most part I do believe the intent of the author is to sorta mess around with the ideas and perceptions surrounding gender. In the case of Reika I'm fairly confident this is the direction the character's suppose to take. If however the author decides to dump some exposition on Reika's exact view of Chiaki in later chapters (as of ch. 24 this is not the case) that shows I've made a mistake, then I will update the chapters to reflect that.