First of all, jet planes don't cruise at 3000 m above sea level, more likely at 11000m. And u tell me the PLAAF keeps 500 J-10 tthat only cruise at below 3000 m?

The problem is the Max take off weight in tibet is lower. The J10 canard actually can at least partly offset that disadvantage

The highest peak in HK is 957 M above sea level. I'm sure one can spot all fighter size objects under 3000 m with a pair of binoculars