It's clear now; the Fourth-tier Contractors' mission is also related to the Shapodi Archipelago. It's unlikely that their task involves killing the Celestial Dragons, as such 'anti-human' missions have a difficulty level that only Su Xiao's promotion/mainline dual mission could trigger.
When Su Xiao killed a Contractor in Dressrosa, he received half of the opponent's current reputation as a reward. If this mechanism hasn't disappeared, the situation in Zone 21 makes perfect sense.
Su Xiao stood up and left the restaurant. Although he was very interested in Zone 21, he now had to go to a more important location.
Zone 1, which is the venue for the population auction, is also Su Xiao's designated location for assaulting the Celestial Dragons.
A large number of pirate ships are moored in front of the port in Zone 1. These people are pirates and slave hunters. They pillage and capture humans, fishmen, fur tribes, etc., to sell them at auction for a profit.
