Is it true the only way to get hidden abilitys in sun/moon is to SOS chain?
You need the Pokémon Bank application for the 3DS family systems. There was an event to get all three starters with their Hidden Abilities. The event started on March 9, 2018, and ended on October 31, 2018. The only way to get them now is by SOS chaining.
As of now, you can't get Popplio's hidden ability in any way except through hacking.
u need to have poke bank, but the event is over anyway. once the gen 8 games are released you may be able to use pokebank to transfer to gen 8 and they MAY return the old feature/glitch (were not sure if it was ment to happen, since it was removed) where when you transfer from certain games before gen 3 to another game after gen 4, the starter's ability would be its hidden ability