This noise is strictly about tones, and a lack of uniformity in tones. When you look at an affected shot, you’ll see patches of bright and dark tones in areas that you know should be the same colour, or taking the same graduated pattern. That grainy look is also luminance noise.
The only way to even this out is by blurring. The issue is we perceive sharpness in this same channel, so blurring to fix the sky will also blur our edges. You’ve two tools to help: Contrast, to retain the contrast along edges and keep sharpness; and Detail, to try and retain highfrequency areas in the image.