He doesn't sound surprised there, he tanked Gojo's sneak attack easily "with just losing two reinforced hands when taking a direct hit".
However, he is overestimating Purple because of that. He thinks Gojo buffed the attack to 120% with a binding vow and he thinks that's what he tanked. He doesn't know Gojo's sneak attack Purple was buffed to 200% with the use of external help from multiple people. In turn, this makes Sukuna be more wary than he should of the technique.
You picked the worst possible panel to hype Gojo and Purple.
"tanked" losing two limbs isn't tanking an attack it's called surviving an attack - "I GOT AWAY" Sukunas own words... you also forgot that "direct hit was just under 4km" it was launched at 2.5 mi away traveling that distance to meet sukuna..
sukunas reaction speed and curse energy or domain amplification enforcement is what saved him here, thats not tanking anything...
if thats the case we can say hanami also tanked purple when he looked worse than Toji did by also reinforcing his cursed technique and protecting himself...
even so gojos non 100% purple did this to Sukuna while also destroying Mahoraga in the process & maho taking the blunt force of purple first:
so he wasn't underestimating it cause if a 100% purpe did connect sukuan would look much worse than this...