The American public is told repeatedly we have the best intelligence services in the world. If that's mostly true, then I have no problem believing General Goldfein isn't just flapping his gums.
One day the American public is being told that we have the best intelligence services in the world, the next day we are being told the Russians & Chinese are hacking & stealing sensitive info practically at will,,,, so which version is true?
I'm sure he does have access to J-20 data that's not known to the public, but most likely the info is incomplete at best. Previous high ranking US military officials assessment regarding J-20's timeline turned out to be completely inaccurate, so personally I wouldn't trust their assessments completely.