Sabo, just like Ace, is a heavy fire user. Sanji is a fire user. Thus, it's normal that Sabo got shat on.
It's the same vein as : Big Mom is food related. Sanji is too. Thus, it's normal that Big Mom got shat on.
It's the usual Oda's Sanji curse 🤣
The curse isn't just his lighter that killed 3 people.