And every open vessel that has no cover fastened around it is unclean.
Torah Temimah: Sifrei — Why does Scripture say “it is [unclean]”? Rather: the same [vessel] that saves itself with a fastened cover in the tent of a corpse is the one that saves itself with a fastened cover in the tent of a sheretz.
English: Why does Scripture say “it is [unclean]”? Rather: the same [vessel] that saves itself with a fastened cover in the tent of a corpse is the one that saves itself with a fastened cover in the tent of a sheretz.
Extension of the protective effect to another source of impurity.
The same sealed cover that protects a vessel from corpse impurity in a tent also protects it from the impurity of a sheretz.