Iraq had air defence too... didn't stop the USAF...
And anyway it isn't going to do much to prevent the use of MLRS either! I don't think they're likely to try any human wave type stuff these days, was only really in the beginning last time - and even back then US/UK artillery was a big problem for them - these days an artillery observer can take out an entire 1km grid square in one go if needed.
I'd assume in that situation he is talking about that NK's air defences (which are somewhat ageing) would be supplemented with more upto date Chinese systems - not sure how effective they are against the US's current aircraft - I know there has been a bit of back and forth with Russia, with Russia being more advanced in aircraft and anti-air tech where China has traditionally struggled and Russia in need of upgrades for their navy especially engine stuff.
The problem with NK is that the terrain naturally helps the defender - it is much harder to use artillery or airstrikes to support a rolling offensive and easier for them to hide their AA defences away so they can't so easily be engaged from range.