Historically low rates means people are buying up us bonds even at crap returns. If China dumped us bonds and depressed the price, it would only make the value proposition that much better for the current bond investors so they would gobble them up and push the price right back up.
Rates (i.e. yield) and price have an inverse relationship. If China were to sell their bonds, this should reduce the price of bonds (i.e. increase their yield). So, regardless of what we think about the likelihood of this happening, or the magnitude of the impact, 'historically low rates' do not provide any backstop.