You're right. He wouldn't be cheating on anyone cause he's single. But think about it like this.
Sakura is under the impression she was going to be abandoned and that he was with Fumi anyway. So how would she react seeing the two together and after everything that was fed to her?
"Ah, they're together. Just as expected."
It would only validate what she took out of context along with what was told to her. Even if Shinobu explains, what reason would she have to believe if he's dating Fumi? None.
Now then, if Shinobu is left on his own, getting closer with Fumi of course but not pursuing dating. How would that look?
"Huh? He's not with Fumi? But I thought..."
It would make Sakura question her decisions to have done what she did. Being a hint or a path to her learnifn the truth of what happened. If Shinobu was to go through with it, that'd basically be the path of Sakura, and fuckboi and Blondie, in being valid with what they thought was true.