A “no accomplice” game is no fun if people can still lie to that degree. Had there been a conspiracy of any kind to lie about Nanjo’s movements, they would’ve immediately been labeled as accomplices. They’re not idiots - given the circumstances and Kyrie’s own words, Nanjo would’ve been the obvious suspect. Not speaking up makes them undoubtedly guilty.
In other words - given there’s no room for lying. Genji, Kumasawa, Shannon and Gohda are all people that should’ve been able to verify Nanjo’s movements. Just one of them lying (regardless of their intentions) and then going with it after the revelation about the murder makes them an accomplice.
Think of it like this. You give a nice white lie, thinking you’re helping a friend out - say that he was with you at a certain time. And then you find out that someone was murdered, your friend is a suspect, and he says to the police he was with you at the time of the murder - the time he asked you to lie about. If you confirm his story, you are an accessory. And in this case, had they been lying for Nanjo’s sake - who in this theory is the culprit - they are accessory to the culprit and his plans.
That’s basically what the definition of accomplice also includes.