I can clearly see that they have used noise effects with a filter!

I suspect this contributor has been uploading for years, before Adobe set new standards or changed their policies. Gradients and wallpapers in general can be produced for free by virtually anyone these days, and hundreds of thousands are available for free (and certainly less expensive) stock sites. But I'm only guessing. 

Ours is not to wonder why, I guess. I gave up long ago why people who produce AI are still managing to sell images of people with six fingers. Or why they were accepted in the first place. Welcome to stock.