This is not legal advice. But I seem to remember that joint copyright is a thing in some creations. Check if you have such a right or not, and what it entails.
And if you do, you could release the works on your own account.
Being "co-author" on Itch is not a legal term.
Another beast entirely is removing credits from a work. If your former team member does this, there might be nasty legal things you could do. If credits are still there, you need not be "co-author" to point to that work. But pointing to a work on another developer's site that you had a dispute with, might not be the best idea.