I'm not sure why people ignore the fact that Koby has incredible powerscaling, him lacking confidence in himself in combat doesn't mean he has ever been weak or had middling potential. We know that when characters lack confidence in themselves they never bring out their full potential. He has always had more potential than just about any character other than Luffy. The one thing that did help him is the old man mentor which is a clear sign that Garp saw that Koby would surpass him one day.
1. Learned Soru in just a few months.
2. Had no prior combat training or experience before joining the Navy.
3. Mastered Rokushiki over the 2 year timeskip
4. Pushes himself beyond his limits ever since he started training with Garp.
This is also why Koby will end up a far better overall character than Usopp, once we see him more confident and throwing hands he will be a monster.
1. Learned Soru in just a few months.
2. Had no prior combat training or experience before joining the Navy.
3. Mastered Rokushiki over the 2 year timeskip
4. Pushes himself beyond his limits ever since he started training with Garp.
This is also why Koby will end up a far better overall character than Usopp, once we see him more confident and throwing hands he will be a monster.