Presumably she's referring to her first proposal -- a carbon tax. It's my understanding that there's nearly consensus among economists that a carbon tax is the most efficient solution to global warming, but political consensus that it would never happen when framed as a tax.
If you don't know what a carbon tax is, or why it's orders of magnitude more efficient a solution than "eat less meat" I'd encourage you to look into it a bit. Essentially if you create a system where what is best for the planet is also the cheapest course of action (for individuals and businesses), you no longer have to rely on convincing every single individual to change their morality/beliefs.
No sovereign is going to agree to subordinate itself without all others doing the same
Classic prisoners dilemma
They are universally rather timid in their scope, and politically fragile. So maybe they aren’t going to get us there, at least not in their current form.
But it’s going too far to say they just don’t exist or can’t exist.
We have plenty of treaties that inhibit states. Nuclear weapons, slavery, pollutants. Enlightentment and enlightened self-interest isn’t out of the question.