Funny, on how APA (i.e.Dr. Kopp) make such wild claims without access to any 5th Generation Designs.
Kopp is quite respected by Analysts. But he is not the Bible of aviation. So, while some of his analysis is quite true some may come across as biased as we see in his criticism of the F-35.
The J-20, with so little information available, will come under obvious speculation because it seems to be more developed than the T-50 in external stealth for obvious reasons. Now we don't know to what level T-50 will mature too and neither do we know to the extent J-20 will mature as viable fifth gen platforms. So, crediting the statement that J-20 could be superior in stealth to T-50 as a paranoid man's delusions is discounted. He could be right for all we know.
There is nothing wrong with the F-35 version that they are getting. His worries are mainly in the WVR domain against fighters with better kinematic performance. But, the Australians don't have an enemy.