How do you know ? Are you the developer of retroarch or something ? I talked with Duckstation at Discord and they said no comments and have to asked retroarch team and they didnt say anything about it.
First, anyone can be a developer of an open-source project.
Second: Duckstation's lead (and the one who basically does the bulk of actual core development) pulled away from Libretro and has left the lr-compatible core in a state of limbo. It's been forked,
but no commits since that happened. Meanwhile, said lead developer has expressed disinterest in porting the standalone/only-current-updated version to Switch. He's not working on a Switch port so we can't have the upstream version, and no one's working on the Libretro core to help that work in general, let alone on Switch.
As you can imagine, no one's happy about this state of affairs; especially since Duckstation is
made to be run on 64-bit platforms like a Switch but just...
won't. As far as Swanstation is concerned,
there is some work on rebasing the project. Maybe afterwards there can be talk about adapting it to Switch. But that will have to wait until after, since it would be a waste to contribute to an abandoned repo that's going to be nuked soon.