That’s pretty much what the CLA says, albeit in legal language: “I have the full copyright of my contributions, and I agree to give those copyrights over to the community”. The way that we handle the CLA in GitHub is designed to provide an audit trail of the signature process.
Stepping back, none of this is unique to TiddlyWiki, and we can learn a lot from how other projects are handling it, which is the thinking that led to us starting to use a CLA in 2011: at the time, it was the standard practice for serious open source projects.