["Follow me for the next few days, and I will personally teach you several Advanced Thieving Techniques through 'Hunting'," the bandit leader said with a smile.]
From this, it seems that the other party probably doesn't have any extra Divine Arts stones for him to use, so they have to teach him in person while he learns by observing.
This kind of freebie lesson is something Mu You certainly wouldn't refuse.
The only thing he found strange was, what was this person plotting?
He was just a newcomer, an outsider. From the other person's perspective, his background and origin were all unclear. Under such circumstances, the other person suddenly wanted to make him the second in command and to impart his own cherished secret techniques...
Mu You was not naive enough to think that the leader of a thieving organization would be so selfless. This person must have ulterior motives.