IIUC, this information you're referring to is whether the author of the pull request has signed the assignment?
Yes, sorry :) IIUC myself of what CLAHub is though, the copyright assignment will have to be binding though CLAHub’s services. That is, I presume some sort of text field with the contribution agreement and an ‘I understand and agree’ checkbox is provided by CLAHub. Once the contributor agrees to the CLA, this agreement is reflected visibly on the PR discussion page.
I hope this would not boil down to a ‘CLA’ saying essentially that the contributor has completed the (separate) FSF copyright assignment. This would degrade the service as a mere convenience rather than an open door for future contributors.