The Torah says the ox gored "a man-servant or a maid-servant." The Mekhilta asks: which kind of servant? This must refer to a Canaanite bondservant, not an Israelite one.
The proof comes from the payment structure. The verse concludes: "silver, thirty shekels, shall he give to his master." The money goes to the master — the owner of the dead servant. If this were an Israelite bondservant, the payment would go to the servant's heirs, not to his master. The fact that the master receives the compensation indicates that the dead servant was his property, which is the defining characteristic of Canaanite rather than Israelite servitude.
This distinction matters because the legal status of Canaanite and Israelite bondservants differed fundamentally. An Israelite bondservant was a temporary laborer with extensive rights. A Canaanite bondservant was treated as property. The payment for a killed Canaanite servant went to the owner as compensation for property loss, much as payment for a killed animal would go to the animal's owner.
The Mekhilta's identification of the servant's nationality through the payment recipient illustrates a broader interpretive method. When the Torah does not explicitly state a category, the rabbis inferred it from context clues — in this case, from who receives the money. The direction of payment flow revealed the legal status of the victim.