Comment by pie_flavor

The CLA assigns ownership of your contributions to the Zed team[^0]. When you own software, you can release it under whatever license you want. If I hold a GPL license to a copy, I have that license to that copy forever, and it permits me to do all the GPL things with it, but new copies and new versions you distribute are whatever you want them to be. For example Redis relicensed, prompting the community to fork the last open-source version as Valkey.

The way it otherwise works without a CLA is that you own the code you contributed to your repo, and I own the code I contributed to your repo, and since your code is open-source licensed to me, that gives me the ability to modify it and send you my changes, and since my code is open-source licensed to you, that gives you the ability to incorporate it into your repo. The list of copyright owners of an open source repo without a CLA is the list of committers. You couldn't relicense that because it includes my code and I didn't give you permission to. But a CLA makes my contribution your code, not my code.

[^0]: In this case, not literally. You instead grant them a proprietary free license, satisfying the 'because I didn't give you permission' part more directly.