The point of regional forms are Pokemon adapting to their environment, not revealing some latent power of their own. With the background of the starters, Decidueye went from being based on a
Hawaiian owl with cultural significance to native culture to a
Hokkaido owl with
cultural significance. Samurott turned into a Ronin, a dishonored samurai, being far from its homeland and becoming a dark-type. Typlosion is an amalgamation of various Japanese spirit motifs and figures, such as hitodama lights, magatama beads, shinigami, and perhaps the main inspiration,
Kamuy-huci, an Ainu deity of the hearth and gatekeeper between the realms of humans and deities.
TvTropes posits that the Hisui starters are a reference to the starter Pokemon available in base Sun/Moon, with Rowlet as the regular starter, Cyndaquil showing up in Melemele's island scan, and Samurott in Poni's island scan.
As for the why Mt. Coronet changed the starters, I'll sooner believe a sacred mountain that PokeGods visited has mystical powers unknown to humans that changes Pokemon with its presences than accept that pancakes give Pikachu telekinetic powers.