It's nice that things haven't changed between them. Matsukawa has had enough change this summer, so he revels in the ordinary routine they all fell back into after day one. They all do.
or
Matsukawa just needs his friends.
He wishes that those words are a lie.
It would hurt a lot less if he is lying about loving him.
But he was genuine and sincere in his confession, even as his activities with other people prove otherwise.
Even so, he couldn't stop loving Oikawa Tooru.
But even if he had forgotten him, will those feelings remain?