So the thing that changed the way Kogahara saw Akiyoshi (from friend to romantic interest) was something Lemon said while in his body? That's rough.
Yes. Explicitly yes. It's the same as with Suzuka, the male friend. He fell in love with Lemon explicitly because of how Sunao acted while in Lemon's body. It's a parallel to how Kogahara only developed feelings for Sunao after Lemon was in his body.
---
As a complete aside, this story seems to have completely dropped the body-swapping and makes no sense IF certain assumptions are true. Assume Raika is aware of the body swap, hence why Lemon explicitly never runs into her on those days when she is in Sunao's body. Assume that in this chapter, it is Sunao in his own body, hence why he was concerned about Kogahara misunderstanding. Assume the president of their film club was talking about Raika.
The president said he saw Sunao with a girl yesterday. A body swap day. When Lemon was inside his body. That makes no sense given the above, that Raika wouldn't be around Sunao's body because that's a day Lemon was inside him.
Its completely possible Lemon was just in Sunao's body and just hung out with a girl. I don't know why.
Its also completely possible the author forgot how their own story worked.
Or maybe yesterday doesn't actually mean yesterday. I don't know.