@Winston The article says that 2.8 million Americans are currently abroad. It doesn't say if they're permanent or travelers or both. 2.8 million of the total US population is about 0.8%. So I guess 98.2% of people in America are happy in America, instead of happier abroad.
I just Googled and other sites say about 8 million - 9 million are living abroad. https://www.aaro.org/about-aaro/8m-americans-abroad
So that 2.8 million must mean legal American residents in other foreign countries.
The U.S. government does not keep a close count of Americans who have left the country; few governments do. The State Department asks some expatriates to register, but it does not maintain comprehensive, up-to-date directories. So, to count emigrants, we need a little help from our friends. Or at least from their statistical agencies.
The United Nations and World Bank collect data on foreign-born populations from local censuses and surveys all over the world, and use them to estimate migration patterns between more than 200 countries and localities. By their estimate, the population of American-born people abroad sat around 2.8 million as of 2020.