Should I click on an article because a "25 year old" has investment advice? Smells like click bait.... and also, the 25 year old's 40% allocation to global is half U.S. stocks, so they hold 20% U.S. equities. The title is only technically accurate, but highly misleading (a global ETF isn't a U.S. ETF - but it still holds U.S. stocks).
I recall a Vanguard white paper on the appropriate international diversification, but can't find it right now. Essentially it said for investors who want a certain benefit, 20% international. For those willing to take more risk of under performance, and wait longer for a possible benefit, 40% international... that was their range, ultimately, 20-40% international.
But if we're talking about what I do in my own market timing portfolio, that's a whole different story.