[maxlybbert]

For the record, I'm not a lawyer. If I understand the question correctly, I believe the argument would be that the contributor believed all contributors were on equal footing (e.g., all contributions had to comply with the same rules) or that the project somehow promised to behave a certain way -- and that promise may have been included in the license. Without a separate explicit promise about how the project would act in the future, I wouldn't expect the lawsuit to go anywhere, but I don't think it's a silly argument, and it may have a better chance than I believe.

Broadly speaking, the CLAs I've seen either (1) fall into the classic legal "belt and suspenders" approach of being explicit where there is arguably an implicit promise, or (2) require the contributor to transfer copyright to the project and then make promises about what the project will do with the code, including promises about relicensing, and a broad license back to the original contributor.

US copyright law sometimes assumes that there is some kind of agreement between people who collaborate on a copyrightable project, and generally speaking, the CLA looks like it serves that purpose.