Why would Ash, Hala, and Kiawe participate in the first place if the prize was a Z-Ring? They'd have no reason to since they all have Z-Rings. The prize is probably something similar to the Charjabug race episode.
Btw, was I the only one reminded of Lana and I'a from SM40 with Lillie and Sara? I dunno, they both seemed to serve similar mentor roles, and both have evolutions of their sigs that are. Hardly anybody thinks I'a having a Brionne rules out Popplio evolving, why should this be any different? Although granted, there at least have been some hints toward Popplio evolving like the PokeProblem segment of that episode and the probable foreshadowing of it learning Sing when it would be far easier to just fill out its moveset with generic moves that we've seen on past Water types (Hydro Pump, Icy Wind/Ice Beam, etc.). There has been no such hints with Shiron.
Also, from my observation, if someone else debuts the evolution of a Pokemon before a main character does, it's usually only when said evolution is a jerk or a rival to the main character's NFE Pokemon that the latter doesn't evolve. Sara's Alolan Ninetales is clearly not a jerk, and who knows, maybe Sara and her Ninetales will have an impact on Lillie and Shiron just as I'a and her Brionne did on Lana and Popplio.