I think Shinobu is already truly in love with Fumi, but Fumi is still feeling insecure. This is probably because even though Shinobu shows his affection, they haven't had those 'I'm over Sakura' or 'Now I only love you' types of conversations yet. During the period between when Shinobu got the director job and had sex with Fumi for the first time, up until he finished writing the script (which I guess might be a month), it seems Shinobu lived together with Fumi (so ShiFumi might have had sex a few times). I think Shinobu is not the type of person who would do that with a girl he doesn't have romantic feelings for. Maybe the timeline progressed too fast and rushed, so we didn't get to see ShiFumi's sweet moments, which in turn makes us doubt whether Shinobu actually loves Fumi.