Rabbi Yonathan builds a towering logical structure to prove that Passover leftovers cannot be burned on the festival — and like Rabbi Yishmael, he argues the Torah did not need an extra verse to establish this. His reasoning uses a kal va'chomer, an argument from lesser to greater, with Shabbat (the Sabbath) and the festivals as his two reference points.
The argument works in two stages. First: on a festival day, all forms of ochel nefesh (the vital soul) — food preparation for human consumption — are permitted. You can cook, bake, and roast on a holiday. Yet even with this broad permission, burning Passover leftovers is still forbidden on the festival. The leftovers are not food; burning them serves no nutritive purpose. So despite the holiday's lenient stance toward fire and cooking, leftover-burning does not qualify.
Second: on Shabbat, the rules are stricter. Only partial ochel nefesh is permitted — certain preparations are allowed, but the scope is narrower than on a festival. If the more lenient festival already forbids burning leftovers, then the stricter Shabbat certainly forbids it. The conclusion is inescapable.
Having established through pure logic that burning leftovers is prohibited on both festivals and Shabbat, Rabbi Yonathan faces the same question as Rabbi Yishmael: what does the repeated "until morning" teach? His answer aligns with the earlier interpretation — it defines the deadline for eating the Passover lamb. "Morning" means the earliest moment of morning, the rising of the morning star, not sunrise. The extra verse pins down the precise hour when the obligation to consume the Pesach (Passover) expires.