Kalylia
Breeder
Have you every taken the time to read any of the books you can read from in the library at Canalave City? There's one story that mentions almost exactly that.
I actually haven't gotten that far in Diamond, yet. I purchased Diamond only a few days before I had to pawn my first DS for rent money. I recently got another DS, and I'm just started to get back into Diamond.
...and speaking of exaggeration... Something like Arceus, Mewtwo, Kyogre/Groudon/Rayquaza or Dialga/Palkia/Giratina could probably pull it off, sure. Maybe even something a bit lower down, like Deoxys or Darkrai, depending on how soon the humans decide to fight back and what kind of weapons they have. But to say that most Pokémon could singlehandedly wipe out all humans is just plain wrong.
Especially in the game universe, where a Dragonite's Hyper Beam isn't even strong enough to kill an average adult human in one shot (as shown in G/S/C, when Lance and his Dragonite fight off some Grunts in Mahogany Town's TR base--the Grunt that gets Hyper Beamed into a wall isn't able to move afterward, but he's still conscious, able to talk, and he recovers fast enough to get up and leave by the time you're done fighting the other Rockets in that base.)
Note that Pokemon is the plural form of the word as well as the singular. I never made any claim that a lone Pikachu could enslave the entire human race. But if a large group of Pikachu were to get together and start zapping a collective million volts of energy through a human, said human would explode like a potato in a microwave.
While Mew on its own could most likely take over the entire human race, and Mewtwo obviously could, as he came fairly close in the movie (but what fun would it be if he'd have succeeded? Then the other 12 wouldn't be in existance), but if a species of Pokemon or a couple species were to band together, it'd be a far cry to think that humans could stop them.
Thankfully, that isn't so much the reality in the Pokemon world.