The outer dimensions are identical. Whether Kia changed the mounting points I do not know. But since it fits, I'm assuming that mounting it could be accomplished easily enough if the mounting points have changed. Yours is the first question about retrofitting the new diffuser and exhaust stubs onto a MY18 that I have seen.
Probably because the giant fake tips on the 2022 model look ridiculous? They wouldn't be so bad if they'd either made em real tips or did a better job in hiding the middle tip like the 18-21 models. When I bought my 2020 I didn't even realize that the tips were fake. Why? Because they don't stand out like a sore thumb like the 22 model.