I've updated the language there a bit, but the meaning is the same.

>There's a fairly good case to be made that playing by, and being able to set, international norms is in the long term benefit of powerful countries.

Yes, but it doesn't matter. The entire concept of truly sovereign states existing is the point. The second a state wants to leave the agreement, they just stop cooperating. There is no "greater sovereign" with which you can enforce these rules, and if there is -- say, a bigger country invading a smaller country to enforce the rules -- you're once again in a state of war, and none of the rules mattered in the first place.

I think it was sort of meant to be a check on the middle powers: sure, you could do some war / genocide, but we have a legitimately-sounding system of rules by which superpowers can punish you.

In practice, superpowers don't apply these rules consistently because it's sometimes in their geopolitical interest for a subjectively less-bad option to win, and they don't hold themselves to the same standard, so yeah, it does end up being a bit of a fantasy.