How do you get to battle frontier on Pokemon emerald?

after you beat the elite four, you will end up in littleroot town. after you come out and birch upgrades your pokedex and take a few steps, Scott will call you and tell you to go to one of the ships. he will be there and will tell you to get off and take the ship to the B. F.

i thought you need to battle the 7 B.F. brains?