I wondered about this too, especially since Wulfric, of all people, got a Hisuian ancestor.
The way I sort of rationalized it was that Drasna’s grandparents are from Celestic Town, but at the time of Legends: Arceus, Celestic Town doesn’t exist yet. So I figured they must have immigrated there sometime well after the events of the game.
Looking at Drasna’s character draft now, I can see that they conceptualized her grandparents as originating from Sinnoh, which — although we don’t know if these drafts are still canonical; obviously there are some details that didn’t make it to the finish line — does kinda shoot a hole in my theory, but at the same time, I always assumed that Drasna was in, like, her 60s, so I’m surprised that she was envisioned as being in her early 40s. If Legends: Arceus is circa 1869, and XY is, I dunno, 2010ish, Drasna would have been born around 1965. And I assume her mother (who the document says married a Kalosian man) would have prrrrrrrrobably been born 35 years or fewer before then, and her grandparents 35 years or fewer before
then (35 being a rough-but-generous estimate after factoring in the average age of menopause + the median age of childbearing in Japan). That’d put her grandparents’ birth at about… 70 years after Legends: Arceus.
So in my head, that creates a nice, long gap in which the region could be renamed as Sinnoh, the Diamond and Pearl Clans could meld, and Celestic Town could be formed. While I imagine that Drasna’s lineage is still ultimately rooted in the two clans, maybe it took a few generations of intra-clan relationships for the genes that make Drasna look as she does to come to fruition.
Well, Palmer mentions at one point that when he was a boy, he and the player’s father went to
Lake Verity, and that that was his first step as a Trainer. So I think
he definitely grew up in
Littleroot Town, though it’s still possible that his parents aren’t from Sinnoh.