Mercedes

He never implied cheating, he said going through a purist mentality, world tour and Masteria are off limits as they don't exist in canon (well masteria is debatable, but his reasoning for that is because it was somehow unaffected by the Big Bang.)

There's another reason, but this one's mostly fan theory right now.

Technically, while it MIGHT exist in the future (via Nexon taking it and upgrading it to be the next plot setting after we kick the Black Mage's arse into next week), it canonically doesn't exist at the moment. That last bit's foggy though, because although it exists in full form in gMS, it is robbed of its true glory in kMS.

However, if they stole the expansion from us and ripped it off, that might be because they're still aware Masteria exists, and they're going over Prose's notes while filling in those blanks, wondering "How the hell are we going to remake this and not have the Masteria purists get pissed at us?". And not to mention, right now they've got one ultimate baddie to worry about finishing (Black Mage) - they probably don't want the stress of having to juggle two villains and not see either one finished in the somewhat unlikely event that MapleStory kicks the bucket (if they go out, they might as well have FINISHED something content wise). And in the event they're still alive when the Black Wings saga ends? They can breathe a sigh of relief and finally get back to work on Masteria - in which case, it now becomes canon for Mercedes to even BE there, as she's finished her job over down south (looking at Masteria's in-game world map, which I forgot actually has one here - it actually would EASILY fit as either the right half of the area where Edelstein is, or a totally new continent up north) and Slimm sends her a request because of her badass history of being a hero(ine) who friggin saved the Maple World once, and now she's gotta do it again.

That's my opinion anyways, and it's fan theory, as Viaje would say. But in the event that this is what Nexon was thinking all along, I will eat spinach for one week if that happens.