I'm not sure it has nothing to do with IP ownership. I "volunteer" to a million of opensource projects. However, I am not giving up IP ownership - they get transferred as part of the license associated with my work.

In the absence of a license, I still retain 100% ownership of everything I have produced. There is a reason why every employment agreement requires to sign a special document where the employee transfer IP ownership to the company. In an "informal" setting, Mark is fully entitled to the intellectual ownership of his work (unless he signed a license or a IP transfer). In the current situation - from a pure legal standpoint - Mark could claim ownership of the project.