>>36497741People found that the wild Pikachu and Eevee also have the state and moveset changes the starters are noted having. Entirely possible this is included in the general moveset updates to the lines. I mean it would be pretty fucking weird if there was two different yet unique surfing Pikachu's in this gen alone, no?
Possible these moves could be learned by other Pokémon in the future. Especially other smaller Pokémon if they want to give move options for surfing in future games. (I am assuming any Pokémon who can learn the move will also have the Surfing field move, just by correlation of the Surfboard. That raises questions about Eevee though.)
Just trying to be fair in evaluating the situation.