The distinction is “one common ancestor” vs. “all ancestors in common”. Despite heavy mixing in the last, say, 200 years - it’s very likely there are ancestors of some 5,000-year-ago American native or Australian aboriginal ancestors who are not also the ancestors of everyone else in the world.
It’s possible, but it’s not likely. Reaching “all ancestors in common” is the same exponential process. They may have been isolated, but were they completely cut off? It only takes a tiny amount of emigration out of those populations to create paths from all of their ancestors to everyone else in the world.