For the time being, and until or unless he tells Obi-Wan, Luke isn't going to touch on the reason that he wasn't fully trained. Given enough time and context, Maul will most likely be able to figure out the real reason.

"Please, I'm no one's Master." He understands that it's a Jedi tradition to take up that title, but it doesn't sit well with him. Maybe one day he'll grow into it, but not yet. "Just Luke. Or Skywalker if you're not comfortable with first names."

"In the fall of the Empire and the rise of a new Republic, I played a role, but so too did hundreds and thousands of others." Modest. Maybe too modest and edging into self-effacing. "If that's what you're talking about, that is."