Unfortunately, more projects should get this “in writing”. GNU projects prefer copyright assignment, but (I think) many of them also accept just a clear licensing statement (“This is mine and no one else’s; I give it under the project license.” but in legalese).

If there’s no clear statement, a judge many very well issue a preliminary injunction against the project distribution, just so they have time to gather and review evidence, even if the actions of the contributor are later found to show intent. With a dated statement containing the contributor identifier(s), a judge is much more likely to accept an initial motion to dismiss (even with prejudice).