As I said in my comment, I assumed the project already held some copyrights, which gives them standing to file a lawsuit over copyright infringement. (As evidenced by the many GPL violation suits by people who hold a subset of the copyrights in a project, such as Busybox and Linux.)
You make a good point about the ability to *not* sue, though; copyright assignment does indeed ensure that the original contributor can't sue, at least not without going through the copyright assignee first. However, the article here points out that Project Harmony's agreements (can) include a covenant not to sue; as long as that extended to third parties, and handled the case where the project has the right to offer a different license than the original contribution, that would remove the need for copyright assignment as a way of preventing the original contributor from suing.
Any other reasons a project would want to hold the copyrights rather than just having an agreement in place?