According to
Portoleau on Foscagno Pogacar pushed more watts than Vingegaard on Tourmalet last year in the TDF (Tourmalet for Vingo was better than Marie Blanque and Domancy). So this narrative that Pog's TDF level of last year was better than the Giro is just wrong. Pog also admitted that his form in the Tour last year was not so good.
Honestly, in my opinion it doesn't make sense just count since the start of the attack.
The performance should count since the start of every climb, because if you do the first part in a low pace, obviously the rider will push more watts in the final part of the climb when he attacks, because he is more fresh.
Besides that, the kilojoule during all the stage is an important area.
Nevertheless Pogacar did 6.15 w/kg on Tourmalet during 36 min and 41 s according to Portoleau, and Portoleau didn’t make an estimation for Passo Foscagno, but according to ammattipyoraily/watts in cycling he did 6/6.1 w/kg in 35 minutes, and 6.7/6.8 w/kg after the attack in 13 minutes, similar with the last 4 km on Tourmalet.
He also didn't needed to be strong in the Giro, like in the Tour to win.
I think he was much stronger in the Tour 2023 and he improved last year, if we compare his level between the Tour 2022 and the Tour 2023.