As long as Sun/Electric Terrain is active, Koraidon/Miraidon get a boost to their base 135 offensive stat by a certain amount.
If that's the case, what would their actual base Attack/Special Attack be with these boosts applied? Is it far more than the majority of the box legends with 150 base Attack/Special Attack?
369 x 1.3333 = 491 (Assuming 252 EVs and 31 IVs in their respective attacking types)
Looking at Pokemon damage calculator, getting an attack stat like that means having a base attack stat of 196.
So yeah. Pretty ridiculously high