The Fed has been talking about U6 since before Yellen took the chair. The Fed's entire dovish stance on interest rates was publicly predicated on low labor force participation being unaccounted-for slack.

The unemployment rate vs labor force participation has been a part of Presidential debates since 2012, and also figured in the 2010 Congressional elections.

If you want to know more about how U.S. government debt is accounted for, the various Federal Reserve websites are informative, if a bit dry.

The China growth rate is probably not far from the truth, but there is not much public data published to back it up. It's reasonable to be skeptical. I think there's reason to believe that their actual growth rate is higher in some years and lower in others, as an artifact of the difficulty in getting accurate numbers.

I'm curious about your comparison of China to the United States. It seems like a whataboutism argument, which isn't really to the point of the topic.