Producers didn't think it through: Highly unlikely. Making an anime requires planning everything in advance. Besides, you can't just put in any new character you want (specially a Pokemon) without getting sued.
It was a teaser: This is more likely, as it showcases new Pokemon to get people hyped for next generations. It's not that rare.
A Pokemon can exist in the Pokemon world without ever being released, obviously, if you think in those terms. Just like in real life. If a species is not discovered by scientists, that doesn't mean it doesn't exist. Still, that should imply a lot more attention towards May's Munchlax, so I assume the NPC are already familiar with Munchlax, or they're too dumb (Highly unlikely.)
Then again, anime logic. This is a standard response that I don't want to elaborate upon, for it doesn't make sense here. Still, it felt obligatory, so there it is.
Besides, the regions are interconnected. This is seen in Ash travelling to other regions (and SPOILER: May's appearance in Sinnoh), so it makes sense that Pokemon too are available from other regions.
To continue, Munchlax evolves into Snorlax, a Pokemon that was introduced waaaaay before, so it's very likely that the pre-evolution, Munchlax too exists. All you need is a Full Incense, anyway.
Anyway, those are just my thoughts: I haven't really watched the episode, nor was I involved in making it (haha) so, I'm not sure why exactly it happened.