Maybe "Red China" realised it was on the same planet as the rest of us, and needed it in good health for its own survival?
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'.
And finally, China is making a ton of money exporting renewable energy technology. So China is acting very much in its self-interest.
Talk is cheap. China isn't reducing green house gas emissions one iota. However, China's leaders are smart and will put pressure on the USA to do so in order to give an advantage to Chinese manufacturers. Trump is calling them out on their hypocrisy. China wasn't the first to push global warming for financial gain, but they sure will if our leaders are stupid enough to let them get away with it.