Chiyou and his servant, Fang Huizhong, had left their kingdom and came into the Xiang Kingdom as an envoy, but they didn't come like some royalties or anything else that told others that they are special.
They came like a normal person as that was what Sun Mo asked them to do, due to strategic reasons.
Because they are right now in simple civilian dress, almost no one can doubt who they truly are and if things go wrong or if they are mistreated by the officials of Xiang Kingdom, they will have more reason to attack the Xiang KingdomÂ
Or maybe... the chances of forming a treaty can arise.
As for why a treaty when Sun Qiang wants to rule the whole world, that is because Sun Mo wants to find a bride for himself from the Xiang Kingdom as King Xiang Fang only has two daughters.
Thus he wants to get married to someone of his status at least before his father rules all kingdom around him.
Though there is more to it.