Fernandez-Villaverde says he goes to each country's national statistical agency's website, and uses their numbers for births and deaths and population, and computes fertility and replacement rate, and the trajectory of population for each country from that.
He says he has no idea where the UN gets its numbers, despite asking several times. He is a professor of economics and demography, so he's not going to just make stuff up.
Even if the headline claim is true (other people have already critiqued it), this would dramatically increase the federal deficit without massive tax increases - and as far as I know, none of the advocates of a M4A-style program have ever explained how they would pay for it.
Sorry Valve. I know its not really your fault but I refuse to give you my money this time. Instead I'm executing plan B: converting my Steam Deck into a workstation. It's the best hardware I have and while it can't play everything smoothly what it can run well is practically endless.