I do think the author is correct to imply that there's substantial survivorship bias when people talk about US market returns over the 20th century. (A stronger statement than the author makes: the causality is bidirectional; strong US market returns are not merely a consequence of the 20th century being "the American century", but a cause of it.)
But equally weighting other country markets is also nonsensical; the lower returns of the Nikkei in the last 20 years go hand-in-hand with that exchange have a smaller total market cap.
But equally weighting other country markets is also nonsensical; the lower returns of the Nikkei in the last 20 years go hand-in-hand with that exchange have a smaller total market cap.
A more meaningful comparison would be to look at whole-world market-weighted returns, and try to correct for survivorship bias, as the authors of this paper have done: https://www.nber.org/digest/jul97/century-global-stock-marke....