That is true. You do have to have a Pokemon that matches the everyday generated number. The secret id is not necessary, the first and second half (that means the whole PID) for this i am not sure. I have been to the island. Once only. Knowing that i have more than 1400 minutes of game-play in Hoenn. The chances are quite slim, and nearly impossible. You have a better chance of finding a shiny or two.
My tip is to have different ID's of you pokemon, and make every trade in=game and others if you have.