More recent v5 versions of PhotoSwipe no longer contain this potentially DSFG violating exception in README. (The copy shipped with darktable of very old 4.1.1 version does not mention this exception, in debian/copyright, However, as the author stated, it is likely meaningless.).
There is some discussion in upstream issue tracker about pswp: https://github.com/darktable-org/darktable/issues/16205 Each "web album" export from Darktable embeds a separate copy of pswp. I am not sure how using a system-wide copy would work, since the generated web album is often copied over to some other server to be hosted, and it might not even be running Debian. So the actual files would need to be included in the file tree generated by the export functionality anyway.
