It's how a lot of open-core products work. Basically, when you hold the copyright, you can apply whatever license you wish when distributing the software at any time. People can use existing copies of the code under their old licenses, but they must follow the new terms if they acquire the code through the new channels.

To get any PR merged in Bitwarden, you are forced to sign a CLA that reassigns copyright to Bitwarden Inc so they can relicense as they wish.

> How on earth does that work? Is that something the GPL license even allows?

GPL doesn't apply in this case, since the copy that you are acquiring is entirely under the commercial license.