Manifold's Github page says at the bottom of its readme:
By contributing to this codebase, you are agreeing to the terms of the Manifold CLA.
With the CLA saying that contributors give away certain rights to Manifold.
But no explicit agreement is required, and it'd be pretty easy for someone to submit a pull request without having read the entire readme.
If the contribution structure is significantly changed, the new one doesn't count. If a court case is settled, if it's obvious how it would have resolved I'll resolve the market based on that, otherwise it doesn't count.
In general I don't think this is a particularly robust way to implement a CLA, but since the open source manifold code is MIT, most of the rights manifold needs are already granted by the license. Then there's the whole question of if submitting a PR implys agreeing to a license in the project repo (probably?). In any case I think it's HIGHLY unlikely that manifold runs into any legal problems with the CLA as is.
@t3ss in summery answer to the question is YES but it has little to do with the CLA