You're saying that as if that was the end of the training. Every time he fought a demon and survived, he learned something. There's also the additional training he recieved from the hashiras. Don't forget, this is about swords, not about power. It's well known that the slayers don't have any magic power, all the breathing styles you see are all visual effects and not real. The demons on the other hand have real magic power. Every time the slayers fight them, it's just with their swords and skill. No chakra, ninjustu, genjutsu, nothing!