Here's the opening paragraph from your link. > Selling exceptions means that the copyright holder of the code releases it to the general public under a valid f

Here's the opening paragraph from your link. > Selling exceptions means that the copyright holder of the code releases it to the general public under a valid free software license, then separately offers users the option of paying for permission to use the same code under different terms, for instance terms allowing its inclusion in proprietary applications. Crucially, Element is not trying to sell an exception as the copyright holder. They want you to sign a CLA, because they want to sell AGPL exceptions code you wrote, and they are not a copyright holder of your code.