It certainly is true, but academics who want to conduct sophistry can use the metric that gives them the answers they want. Comparing USA to Denmark using in-country income percentiles gives Denmark a much lower bar to hop over for this BS "social-mobility" as defined in the article.

You can also use whatever metrics you want, and you haven't even gone to the effort of defining them!

Here's one: use USA percentiles for both countries. Or use Denmark percentiles for both. Only when you use a high bar for US and low bar for Denmark do they come out on top.