The point Buffett and Munger were making is that the current American system works very well and creates enormous amounts of wealth. Doss the fact that there was slavery for centuries all over the world somehow refute that? I can't see how it does.

If you mean "what if time went on without Europeans in the US", that's a big what if. The indigenous peoples would have carried on in the Bronze Age, and would have been wiped out later by a different collection of foreigners.

The reality is that the US benefited from the culture and know how of Europe without the baggage of supporting useless aristocracy.