Aye, mostly. It's important to remember that the Federal Govt. has the ability to manipulate the money supply by printing money and the ability to nearly indefinitely borrow whenever they get into trouble. Note that borrowing implies issuing bonds, which will lower interest rates.
So the Fed cannot simply dictate interest rates, but if it wants to manipulate the rates there is not a whole lot stopping them. In a recession scenario I imagine they would not have much trouble justifying more borrowing to the American public.
With regard to the effect a Chinese selloff would have on the market (both the US bond market and the wider global economy) I don't know enough to comment. However, I am fairly confident that if such a selloff resulted in a major economic downturn the Fed would try to lower rates to stimulate the economy.