Sora did mean he'll keep it to himself - that is, that he won't let it outside at all. Not in front of Soldat, or Misty, or Jason or Naminé or anyone. He doesn't make promises in halves! He knows exactly how big of a promise he's extracting out of the soldier, and he plans on doing his best to match it. No Rage Form, even if he has control. That is the promise Sora made in his head, he has no idea that the soldier is thinking of what he said in a different light.

Which makes this addendum a little more... bewildering, because Sora thought they were both on the same page of never ever ever again, here. "I'll... yeah. Yeah. That's definitely something I want." Because the point is to never have to use it again, right? "And, I mean, I still need to fix what went wrong just in case, but that's sort of." Sora looks down at the gashes in the trees and ground around them. "I. Needed to work out the thing that made it try to stop itself anyway. By myself."

He looks back up at Soldat. He means it. "You don't have to promise anything because I can't promise anything either. I... just wanted to make a point. This is a part of me. It's saved my life. If you don't want me to use it, I either have to get better than it at everything or I need to ignore that it exists when people need me." And he absolutely will not do that. "I don't know if we have time for me to get better at everything. I'll try. I just can't promise that I won't use it again. "