I feel like a lot of people are really getting stuck on the Trump stuff and missing Buffetts core argument. Here's a pretty simple guy:
When the US has a trade deficit with China, it tries to balance that out by issuing debt in the form of treasuries.
As more treasuries get issued to foreign nations, the risk to American sovereignty increases. People really forget Japan's blitz on US real estate and businesses in the 80s and the drastic actions of that era the US took to knee-cap Japan.