You're trying to use a few off-hand small cutscenes to rationalize this theory, all of which are always tacked on at the end to usher the player on to finish the content, and yet in any cutscene where these events take place it is only ever our character that is scene in them. Whenever there's a integral cut scene with voice-acting, and true story progression our character is only ever the sole protagonist. Take for instance, "Stone Vigil" where Cid's ship was last seen, and we enter to with a small party to navigate through it, if the story truly wanted to acknowledge their presence why weren't they in the cutscene when Cid, and Alph approach the dragon, or when we encounter the Ascian? How come we escape on the Enterprise with JUST the 3 of us, and none of the previous party members?
I'll tell you again, it's called "Game mechanics." In every game, it's up to the player when to interpret whether or not game mechanics are canon. In Final Fantasy, Game mechanics are hardly ever canonical, especially when they're inconsistent with the main plot of the game. In which here they are. So you decide, which canon are you going to readily accept. the main plot which clearly states that we are the lone adventurer slaying these beasts, or the game mechanic that's directing the "player" to find a party? It's as if someone was to suggest that health bars, and the ui actually DO exist in the real world of Final Fantasy just because we see it on our screen. Honestly, it's a bit of common sense.
And lastly, Alphs sort of confirms that we are the sole warrior of light capable of these feats during the conclusion of our encounter with the derplander. He claims that he was unaware that anyone besides us were capable of taking on primals, and it would have been terrible negligence on their part had they never even heard of them. This sort of puts the nail in the coffin on this subject.