Nah, even though Naohiko may have had an inkling that Eiji likes Izumi, it was only that - an inkling. He's not a psychic. Eiji never confessed to Izumi, they weren't dating, so Eiji didn't have shit to stand on. Naohiko confessed fair and square, and Izumi said yes. Not his fault that Eiji is an emotionally-constipated wuss who can't muster up the courage to confess.
I hope Naohiko and Ikezawa get out of this mess of a love triangle just fine. They both deserve way better
Edit: I just re-read the prologue again, and Naohiko didn't even know that Eiji liked Izumi because he was always going on about his senpais and older women. Nao had a feeling that Izumi liked Eiji, but when he confessed, Izumi said yes, so ofc he's going to think that he was mistaken re: Izumi's feelings for Eiji. So my point still stands.