The Hatterene and Grimmsnarl lines feel like they were suppose to be version exclusives, with Hatterene being in Shield and Grimmsnarl in Sword. They're found in similar areas (Motostoke Outskirts, Glimwood Tangle, Lake of Outrage), there's a trade in Stow-on-Side that gives the hypothetical other version exclusive, they're counterpart lines like Reuniclus and Gothithelle, who are version exclusives, and their G-Maxs were version exclusive in Wild Area events.
My guess was maybe that was the intent HOWEVER they then thought of doing that funny little skit in Glimwood Tangle where you light up a mushroom and a Impidimp just saunters over to you to start a battle. It's just feels like something a Impidimp would do and not a Hatenna, but wouldn't be fair for one version to have to have both and the other only one, so they decided to just have both in the game.
"But, Game Director, if we do that then the only Gen VIII Pokemon who would be version exclusive would be Applin's evolutions which you just need the other version's apple to get, Fossil parts, Indeedee's genders, the Galarain Variants & Evolutions for Farfetch'd and Corsola, and the Wolf Hero duo. There would be no completely separate com mon families from the new generation"
Hmm... give me a list of the one-offs we're putting in. *Looks through* Cramorant no. Falinks no. Hey, the ice cube penguin and the stonehenge mon, we doing anything special with them? No? Good, make them version exclusives.
"But, Stonjourner is appearing early on Route 4 and Eiscue is appearing later on Route 9."
Find a new neutral place for them, a mountainy snow area, think the last Route we have planned is like that? Yeah, Put them on Route 10.
"Actually in the past we've had asymmetric distribution. Like in Alola we had Turtonator on Blush Mountain and Drampa on Mount Lanakila so we can keep them..."
Well that decision was made my another game director for another game. I'M the one making the decision now for THIS game and I want both version exclusives to be in the same location.
"... We could just keep the Impidimp family and Hatenna family version exclusive..."
THE SAUNTER ANIMATION ISN'T FUNNY IF IT'S NOT IMPIDIMP!... I make good and rational decisions.