That's simply untrue, duckstation changed license from GPL3 to CC on September 1st 2024, while swanstation retains the original GPL3 license from September 11th 2019.

So the beef is that they kept using the code as they were before the license change, which is their right under the original GPL3 license.

~~If anything is legally questionable here it is the duckstation re-licensing to CC because the author of duckstation is not the author of PRs made before the change to CC, thus they might not have the legal right to change the license to those parts of the code without the assent of the individual PR authors (in most jurisdictions I'm aware of at least). I didn't see any Contributor License Agreement in the repo, which would be the usual way to acquire this assent.~~

Edit: Context somebody posted upthread. They rewrote parts of the code and got some contributors to agree to the license change. Remains unclear if that covers everything even to the author apparently, but fair enough I guess.