It's just simply the perfect VAM climb.
Peyresourde was on the back of Balès, 600 m higher, and with 1.2 percentage point lower average gradient.
I realize that, I even posted about this yesterday. Still, amazing numbers. It's not the only perfect VAM climb done by top guys in recent years. Compare to PDBF last year - Pog and Vinge (both clearly in form) managed 1860 m/h for 20 minutes, also Pogacar managed 1850 m/h on Eze in Paris Nice (but for 15 or so minutes) and 1890 m/h on Carpegna for 19 minutes (it was 300 meters higher but 10% steep). And this is 2-3 percent faster and 20-50 percent longer.