It's not a question of fact, but of definition - to what group do we refer as "mammals"? So it makes little sense to argue over it.

All groups of animals defined via shared characteristics have fuzzy borders. The fact that all extant species either clearly do or clearly don't lactate is purely an evolutionary accident, since all the intermediaries once existed. Even today we have species that lactate but don't have localized nipples (the platypus exudes milk from a wide area of skin). Farther out, we have various fishes, amphibians, etc. that exude specialized non-milk substances from their skin for their young to eat. If you throw away the categories, "feeding the young on substances released from skin pores" is a wider category than just mammals.

Also, this whole thread started with people saying that "common people" wouldn't know if dolphins are mammals, even though they do lactate.