They're there because Hisui is ancient Sinnoh, so PFQ considers them part of the Sinnoh dex (rather than adding a whole other region for just a few Pokemon). Although it does confuse me a little with that and Paradox Pokemon- if Wyrdeer is considered to be from Sinnoh even though Stantler is from Johto, why shouldn't Scream Tail be considered to be from Paldea, even though Jigglypuff is from Kanto?
I assume it is a similar case to regional forms.
To explain, as we know Obstagoon is in Galar while the Galarian Zigzagoon/Linoone are still marked as Hoenn due to the originals being there. Meanwhile, at least in PokeFarm terms, the Paradoxes are alternate forms similar to Megas, thus remain in their original slots.
However, I will discuss a few weird ones to me, being Wyrdeer (though SV does have lore that Stantler slowly lost the natural powers it has to evolve), Ursaluna (though Bloodmoon's entry gives implications they died out) and Kleavor (which has the least amount of excuse now thanks to Blueberry) in Sinnoh.
I will excuse these better though than the fact Dundunsparce, Farigiraf, and Kingambit being in the Paldea region dex. Those aren't from regional forms and there is no "they could only exist in the past" excuse, they shouldn't be there. I'll let Dipplin & Hydrapple's placement slide because of putting Kitakami with Paldea and it'd be bizarre for a line's evolution to go from Galar to Paldea to Unova.
TLDR: Scream Tail = Regional form. Regional form = Connect to base mon. Wrydeer is a new evolution and a case can be made in either direction, with the current way likely being for consistency in terms of Hisuian evolutions.
