There were various articles about this recently and not all pollution is created equal. Local high ppm counts might be a problem for the local population and look absolutely insane, but those won't create a global warming effect.

Blowing CO2 etc. into the atmosphere doesn't look bad in your postcard pictures, but actually adds to the climate problem.

Don't conflate the two things, 'covered in smog' doesn't necessarily mean 'worse than my industrial area with clean skies'.

Chinese coal-powered generating plants produce pollution that causes huge health problems, and also put CO2 into the atmosphere. Also, China cares about global climate change because it has a big desert on its Western side that is spreading.

And finally, China is making a ton of money exporting renewable energy technology. So China is acting very much in its self-interest.