What if the U.S. Complies with the Paris Accord Obligations?

This is from law prof Eric Posner:

I’m told by someone who knows about these things that the Obama-era climate actions, combined with ongoing shifts in the U.S. energy sector, will ensure that the U.S. will reach its announced (voluntary) target under the Paris agreement, or nearly so, whatever Trump decides to do. Rex Tillerson has also chimed in:

I don’t think we’re going to change our ongoing efforts to reduce those emissions in the future either, so hopefully, people can keep it [the Paris withdrawal] in perspective.

If all this true, then U.S. policy is to effectively comply with an agreement that the U.S. formally repudiates.

Assuming this is true, to what extent would it undermine the GATT Article XX legal rationale of any attempts to impose carbon tariffs on the U.S. for pulling out of the Paris Accord?