Yes. Every AC game was that way. 1 Island.
So yep.1 Island per console.
The main difference was you could use a seperate memory card on the Gamecube to use different islands.
They should have it tied to profiles in this day and age and hope they change it, as the way it is with current system structure (no memory cards) just doesnt make sense.
Doesnt affect me but should still be changed.