I might have mislead you a bit, as Lensfun moved to github. Maybe the FAQ list will help you further: Lensfun ?!? (the supported lens list seem to be the same in what Samyang is concerned)
Just have a look in the lensfun files from your drive. There should be at least one having Samyang in the name. If you find your lens there, than there is something wrong with darktable. Otherwise, maybe your lens is not in the ‘version 1’ of the DB, but only in ‘version 2’ (lensfun/data/db/slr-samyang.xml at master · lensfun/lensfun · GitHub).