At my work the guy who wrote pretty much all of the code can fix stuff far faster than me. But then it's not documented, not tested and he is really unhelpful a

At my work the guy who wrote pretty much all of the code can fix stuff far faster than me. But then it's not documented, not tested and he is really unhelpful at explaining it to others. Is that really a 10x engineer, when the fact others have difficult is down to the way he works?