They don't -- at least not directly. This lend less policy is for Chinese banks lending to Chinese citizens and businesses. They didn't tell US banks not to lend to US citizens
Indirectly they have some influence on government policy in that they can stop buying US debt if they feel that the debt is starting to become too risky but that is no different than any other country and it is not limited to debt. Canada can stop selling China stuff or stop buying stuff exported from China as a way to influence Chinese policy. That rating companies view the NDP as a major credit risk influences our policy in that they could never form a government without increase Canada's ability to borrow money. This is just globalization. With China holding so much debt it is a little scary but the reality is they can't really hold any other currency so there is no threat of them torpedoing the USD.