That just means that the method isn't something that's existed in the games before - there are several new Gen 9 Pokemon that have an unknown evolution method as well. Considering that several of the other Hisuian mons such as Growlithe or Sneasel have evolution methods that we actually do know, that's very likely not the case.
Basculin-H specifically was changed to evolve into an invalid index number instead of Basculegion, and Basculegion specifically has egg moves defined (unlike any other evolved Pokemon), so I think there's a bit more credence to it being an outlier.
Scyther and Ursaring evolve by using items introduced in Legends. Stantler, Qwilfish and Basculin evolve by using a move X times for inane grinding reasons. The other Hisuians evolve by using already seen items.
Honestly, just introduce the Legends items, change the "use X times" into "know the move", and stop enforcing artificial scarcity.