It contains 3 Play Boosters, which contain cards legal in the Standard tournament format, the scene cards itself are Eternal legal only
The budget-friendly versions of this exact effect are dirt cheap and stack: Marauding Blight-Priest ($0.25), Epicure of Blood ($0.34), and Defiant Bloodlord ($0.39)
So as long as development tested the Rats in high numbers to make sure it wasn't degenerate, we knew it was safe to break the rule (well, as safe as development can ever be the real world just has so many more playtesters.) Rule #4 Breaking Rules Is A Test For the Future When we do choose to break a rule, R&D tends to keep an eye on the card
Sure, you need to attack first to unlock the ability, so its not quite as immediate
The key here is to have enough mana to cast Kangee at a premium