The numbers you posted are the US's federal government debt to GDP and Chinas total overall debt to GDP (including private debts such as mortgages). The two numbers are obviously not comparable at all.
My understanding is that analysts frequently include not just the debts of China's central government, but also all of the provinces, which are much much more heavily indebted. Under the argument that the Chinese central government is going to cover their provinces' debts in a pinch. (Not defending the specific numbers quoted by OP, just a general observation)
The 77% is total government debt, which includes provincial and local government debt - national government debt is only about 10% of GDP. The 280% figure includes corporate and household debt.
China's government debt to GDP is 77%, vs 121% for the US : https://www.imf.org/external/datamapper/GG_DEBT_GDP@GDD/CHN/...
The numbers you posted are the US's federal government debt to GDP and Chinas total overall debt to GDP (including private debts such as mortgages). The two numbers are obviously not comparable at all.