That is correct. Server default value (for new accounts/characters) is XP 2 (double Blizzlike rates).
During bonus periods you need to change it to XP 4 with the command: .xp 4
You only need to type it once and it will remain at XP 4 as long as the bonus rates remain active. Droprates etc remain server wide, so even without typing .xp 4 you still benefit from everything else, it is only the XP that doesn't benefit.
Once the period of double rates is finished, it automatically defaults back to XP 2. Setting XP to either 0 (no XP gain) or 1 (normal Blizzlike rates) with .xp 0 and .xp 1 respectively will set those rates, and those rates will remain at that setting until you choose to change it again.