It just sounded as if being public domain somehow makes project more ambitious (that is, means to be more difficult to complete than if it wasn't public domain), so I probably just misunderstood.

Except that some jurisdictions in the world don't have the concept of public domain, so, in those places, this code is legally in a very weird place. It's much better (IMO) to assert copyright, and then release under the most permissive license possible.