Thanks for pointing out the mistake, just fixed it now.
Probably the anime. The next couple of star bosses will probably take at least 3 chapters - they have to beat the rebels, then the two bosses, then backstories for the bosses and the remaining explanation about team star's origins, then maybe the 'big boss' will show up immediately afterwards. In real time that's three months. The anime meanwhile releases weekly so they can churn out 12 episodes in the time it takes Yamamoto to draw 3 chapters.
Well, in the game the legendaries can't really go on a rampage since you catch them immediately after freeing them, I'm guessing they could actually be dangerous here, considering they are sealed for a reason.