At that earlier point, like in the classroom, it's probably more of a "happy to see you" thing going on between them and they're glad to be in each others company again, but there's an unspoken tension knowing what they've done with the MC and it's not totally back to normal for them. They still hadn't fully resolved things which is what that scene at the party was all about (albeit not done very well).
Even then, ep 5 reveals that they still haven't been completely honest if you go down the friends path because they each did want "something more" but were worried about what the other would think if they said so. When he chooses to be friends, they don't speak up because they're worried the other one won't react well and that the MC will still choose friendship thereby ruining 2 relationships for them, and if he chooses but they reject (DIK path), both of them are simply scared to admit they want something more in case the other reacts badly.
So even if they are back together, they're still not communicating properly, and this is what will be their downfall on the friends path because they're not being honest about their feelings. Whereas on the throuple path, he'll be like the missing link in their chain, not just an equal part of the relationship, but the one who is essentially their "rock"; keeping them grounded, being supportive, and making the relationship strong.
