Because Terra's power at the start of the game when she didn't even know she was half-Esper is completely comparable to her power after she comes to terms with her identity.

Oh wait.

That's not even a particularly representative battle because it's programmed to end as soon as Sabin hits Vargas with a blitz, no matter how much damage it does. (Actually, in theory a powered-up party could defeat Vargas before Sabin shows up, but I'm pretty sure it's also programmed to throw them off the screen after they do a preset amount of damage to him - which, let's note, isn't him killing them, or even damaging them; it's just him removing them from the battle). None of this speaks particularly well to either Sabin's ability to damage people who are not Vargas or Vargas' ability to damage the party. All it speaks to is Vargas' ability to push opponents out of the field of combat, which wouldn't contribute anything to a cage match even if he were a participant.

And let's be honest. Sabin isn't the one who fried fifty of the Empire's best soldiers in five minutes.