We can't be sure of anything on the J-20, but it's doubtful the plane's technology can match the F-35. Here's a short quote from USAF Chief of Staff, General Goldfein, and he isn't impressed with the J-20.
Let's be honest though, could he have said anything else of a much different tone?
It isn't like he could say "oh yes, the Chinese have produced a very competitive aircraft that will likely place pressure on our future air combat capabilities"...
It's not like any military would admit something like that whether it was true or not; hell they probably wouldn't admit it especially if it were true, and that goes for everyone, not just the US. So with all respect to the General, I consider his statement to be not only expected, but also dismissable.