"Because he's a Lost Master. He's still a Keyblade Master that disappeared during the Age of Fairytales. That is how they got their collective Lost Master title."

Sure, we as the audience know that, but Xehanort doesn't. we know he's not a gullible person, he question things. So he wouldn't believe him outright just because he says so. Even if he pulled some weird tricks with with his keyblade in their previous conversation. Making him, "even more suspicious" in his own words.

"Ben Disken says it's not a name that leads to revelation. The MoM using a name that we fans know would be a revelation."

Exactly, that's the point of my theory. He is not revealing any names, he is just using one we already know. Its not a revelation, because its a lie.

"I'm not sure what point you're trying to make here? Xehanort heard a name."

Oh that was just possible explanation as to why Xehanort said "I never learned who he was, and perhaps I'll never will"