It means previous versions remain open, but ownership trumps any license restrictions.
They don't license the code to themselves, they just have it. And if they want to close source it they can.

GPLv3 and copyleft only work to protect against non-owners doing that. CLA means a project is not strongly open source, the company doing that CLA can rugpull at any time.

The fact a project even has a CLA should be extremely suspect, because this is exactly what you would use that for. To ensure you can harvest contributions and none of those contributers will stand in your way when you later burn the bridges and enshittify.

Licensing the source as GPL doesn't really force the copyright holder (which is 100% BitWarden due to their Contributors Agreement^*, no matter who contributed the code) to do anything - they are absolutely free to release binaries built on the same codebase as proprietary software without any mention of the GPL.

For example if I write a hello world terminal program, release its source code under GPLv3 and then build it and give the built binary to you (and a permission to use it), you cannot force me to give you the source code for that build because I never gave you a GPL licensed binary.

If you were to take my GPLv3 source code and distribute a build of it however, you would have to license your binaries under GPLv3, because that's the terms of the license I provided the source code to you under. Your users would then have the right to request the source code of those binaries from you. And if you released the build under an incompatible license, I (but not the users) could sue you for violating my license.

Their previous versions, still being under the GPL, would require them to release a change to make it usable on desktops.

License violations are usually not resolved by making the violator comply retroactively, just going forward. And it's the copyright holder (so BitWarden themselves) who needs to force the violator to comply.

^* this is the relevant part of the CA:

By submitting a Contribution, you assign to Bitwarden all right, title, and interest in any copyright in the Contribution and you waive any rights, including any moral rights or database rights, that may affect our ownership of the copyright in the Contribution.

It is followed by a workaround license for parts of the world where copyright cannot be given up.