Intent and custom both matter quite a bit in law. It is customary to treat the name attached to a commit as the copyright holder of any changes represented by that commit, just as it was for the sender of an email containing a patch back when that was how such work was done.

Often this is also spelled out in a project’s contribution guidelines, and some projects have even had more explicit copyright assignment policies they required contributors to agree to, but the lack of such guidelines or assignment policies does not mean the custom as normally observed in the field is irrelevant.