@silverden
Sure, with raw power. But technique and form are completely absent from his knowledge. Most martial artists would see the gaps, so him being an expert isn't a very good lie. A better response would be that he is strong enough to brute force things as good as most martial arts would be. And that his magic is strong, using the sage again as a cover-up. Then again, I dont think many would even care about the details behind his power once they saw it, given that any semi-believable vague response would work.
People trying to hide something tend to overlook the fact that most people are just curious. Just give a response which is vague but believable, or points towards something/someone who cant be easily checked. Example, a dead person. Mention a dead person who was "close" to you, and people immediately stop digging deeper into it out of respect. If they ask you eventually about said dead guy, just say they were very strict, and kept their past to themselves. All you know is that they were very strong. How did they die? Old age is a good one, but illness/injury can also work.