I think we should treat Kryptonite resistance as a form of Radiation Resistance, instead of considering it to be Power Nullification. I get your argument for considering it power nullification, because of how the statement notes that Kryptonite interferes with his abilities and weakens him. However, I still disagree with classifying it as Power Nullification. Superman is powered by a specific form of radiation [solar] and Kryptonite limits how much he can take in. But it’s not like Kryptonite is directly weakening him according to your statement, it’s just messing with his metabolism of solar radiation.
Here’s an hypothetical analogy that I hope explains my viewpoint. If you have Character A who gets his powers from X but is also allergic to Y, which slowly infects his powers from X, this doesn’t mean Y is nullifying his powers. It means the fictional allergy is a weakness of his physiology.
Terrible hypothetical situation aside, there are pages for
other comic alt universe versions of Superman, that treat resistance to Kryptonite as being analogous to resistance against radiation.