The speaker they had on said that China saying they won't increase further means they're going to start using the other tools in their arsenal, which in a trade war is not going to be nice.
China owns a bit under a trillion USD of US Treasury bonds, that is to say, debt. If they decide to liquidate it, that's enough to seriously affect the value of those bonds, and therefore the US's ability to borrow more money, and from there, the value of the dollar.
There are a few comments elsewhere saying there are no facts to back it up. Canada has an election coming up and it makes great headlines. Carny has a doctorate in economics (?) and has been the head of bank of Canada and Bank of England so he’s well versed in bonds and currencies.