Marsupials are a primitive form of mammals. Marsupials are a kind of mammal, and they are a kind that is more primitive than placental mammals. The idea that placental mammals didn't evolve from marsupials is entirely irrelevant to the statement that marsupials are a primitive form of mammal.

A mammal is an animal that has mammary glands. Marsupials have mammary glands. Marsupials are more primitive than placental mammals because they don't form placentas. Therefore, marsupials are a primitive form of mammal.