Again, he didn´t go back with Yami once he saw her again. He was being just gentle to get an appropiated closure with her, as much we can blame on him for want be again friend of the right now best friend of Hikari -and yes, is in this way both they returned so quickly to their couple dynamic in this dialogue in the classroom, as friends, before Yuu leaving it for go to Hikari-.
He decided have a relationship with Yami precisely as a way to get over Hikari and can love only to Yami, and she knew it, she says directly it in her chapters she is decided to make Yuu forget Hikari. And he was reaching it until the moment Yami ghosted him and broke up with him.
His actions with Hikari after Yami breaking up with him, are much more coward, questionable and did damage to both, but with Yami, Yuu definitely did nothing wrong.