Legally: There was an Event in Japan that let people get Speed Boost Torchics in the Dream World in Japan. They could have been traded via GTS negotiations.
Illegally: Pokesav, PokeGTS, Action Replay, or PokeGen.
Pokegen or Pokesav so in other words they hack
They probably used AR to hack the game or Pokegen/Pokegts. Also if I recall correctly there was a special event in Japan when B/W was first released, but I could be wrong about that.