Hmm, idk. Looking at chapters 49 and 50, they had plans to go overseas. He says he chickened out and couldn't do it, because he couldn't leave Mitsuki behind. That's when they broke up and she went alone.
This doesn't seem like not wanting to commit to their relationship, but rather not wanting to leave everything else behind to make it big abroad.
Having reread those chapters, I don't think it's entirely fair of Kanna to spring this on Joe. This 'pretend family', as she calls it, would not have been a thing at all, not with Mitsuki, had both of them gone overseas.