The Mekhilta extends the previous argument about festival labor restrictions to Shabbat (the Sabbath) itself, using an elegant reversal of the kal va-chomer — the argument from lesser to greater.
The established point is that on a festival day, your neighbor — a fellow Jew — may not perform labor for you. This has been derived from the verse about the festival. But the verse speaks only about the festival. What about Shabbat? Does the same restriction apply?
The Mekhilta presents the question almost rhetorically: "Would you even ask that?" The logic is self-evident. If you are prohibited from having your fellow Jew work for you on a festival — which is the lesser holy day, carrying lighter penalties and fewer restrictions — then how much more so on Shabbat, which is the greater holy day?
Shabbat outranks the festivals in every measure of sanctity. Its violations carry the death penalty in the Torah. Its restrictions extend to categories of labor that are permitted on festivals. If the lesser day forbids your neighbor from working on your behalf, the greater day certainly does as well.
The argument is so self-evident that the Mekhilta frames the question itself as almost absurd. The very act of asking becomes a rhetorical device — a way of emphasizing how obvious the conclusion should be to any attentive student of the law. Sometimes the strongest legal argument is the one that makes the question sound foolish.