It imposes limitations on how one can use it https://freefilesync.org/faq.php#commercial which I don't believe squares up with its GPLv3 license. Between that, and there being no obvious response to any of the submitted patches or bug reports in the forum, I'd guess this is "fork-only open source" ala sqlite

  $ curl -fsSLO https://freefilesync.org/download/FreeFileSync_13.4_Source.zip
  $ unzip -qc FreeFileSync_13.4_Source.zip License.txt | head
  A.                  GNU GENERAL PUBLIC LICENSE
                         Version 3, 29 June 2007