No I said them getting shinies after Kieran messes with their mind; I never mentioned ryland or ancient times or Leisel or anything even close to that.
One shiny in your life isn't weird in-universe, Melia's full shiny team is just shy of being outright stated to be as a result of Arceus's Light, and unobtainable for any normal person (Our character's shiny rate might also be highly unusual), and Adam is only non-generic NPC aside from Melia to have more than one shiny across all difficulties (Where Love Lies shows that Keta's Ninetails was a gift from Tesla, and I think his Lucario might've been something from his father; talking with it requires a close bond, and he didn't develop that until Chapter 3 in that game, yet it was evolved at Chapter 1). Young and new trainers basically never have a shiny pokemon (Ren, Venam, Aelita, Amber), the weaker gym leaders (1-8) have no shinies (except Keta but unlike the rest he's not a new gym leader), while every gym leader that isn't a replacement has a shiny. Florin's weird because it's only on one version, and that version isn't the default one, and the Intense Major Battles doesn't say that any pokemon are supposed to be shiny aside from Sceptile. Kieran not having shinies might imply a different reason for Adam having 2, and also imply that Kieran doesn't have any Light of Arceus.
Shinies are usually someone's ace (aka naturally stronger than the rest of their team), the dialogue with Vitus implied that being shiny is a result of having Arceus's Light. Plus from Rift Aelita we know that rifts are based on inner power that pokemon naturally have more of (and her rift is a Regirock, and Vivian had the power to create / summon Regirock on the spot), and we know of two rifts being made from shiny pokemon (gardevoir and garbodor), but it's hard to tell if they're canonically stronger than normal rifts or if the other rifts are weak cuz they're early game. Vivian having none, and Aelita not having any shinies on either of her two teams might imply that whatever Vivian had about listening to the world has nothing to do with Arceus (or gaining shinies after awaking the power confirming it).
The 3rd thing is that shinies might've only started appearing after Storm 7; while the game doesn't change your shiny odds in the past (and Melia can find shinies in the destroyed world), no one in the past had any shinies on their teams; the Regis might be shiny cuz I remember Angie piercing herself with a red crystal to summon Regice and shiny Regice is blood red, and Vivian never battled with Regirock, and the golems are called "gifts from Arceus". Gardevoir's "Past Reflections" showed her as a shiny ralts, but Vitus messed with her mind a bunch that it could go either way. We'd need Mr. Luck to talk about his ralts to know if it was actually shiny, but if it was, and that Vitus talked about it's true power in that scene because it was shiny, and him "taking back his power", I kinda think he's the first human that despises pokemon for having more of Arceus's power, since the Narcissa sidequest shows that he never ages and takes on new identities all the time.
It's hard to actually gain any useful information from any of this; at best it'll just help point to important things in the future.
Edit: Thinking about it more, for normal people, it's probably more likely that experienced trainers have a shiny, and people who've explored a bunch have a shiny. The Puppetmasters and Souta have essentially been raised in captivity, no shiny. Ryland is experienced but never left the badlands, 1 shiny. Adam is experienced and spent most of his life exploring, 2 shinies. Ren / Aelita are new trainers only just starting to explore, Venam / Saki / Amber / Crawli / Valarie are young / new leaders that mostly stick to their own towns, no shiny.