This is definitely not true… Sanji wasn’t bulletproof in WCI while his brothers were…
maybe you are mistaking durability for Endurance, I can suppose Sanji had more endurance than them
And if you want to add Armament haki, since his brothers don’t have haki, that’s believable that his armament could have pushed his durability
but if it’s plain resistance, Sanji’s brothers were above him… Now with the same exo skeleton the are even and if Sanji uses armament he goes far above them in durability with Haki
that makes sense right?