i mean i'm not sure what definition you go by, to me the most sensible definition would be "a nutritious liquid produced by the parent to feed the child in its infancy", for which there's actually quite a few other classes that qualify. Many birds have crop milk which is exactly what it sounds like, and apparently some jumping spiders secrete milk from the same place as the eggs come out.

I suppose you could argue that milk must be modified sweat, but that feels very much like actively narrowing it down to only count mammals. It's not like there's anything inherent to how our milk is produced, all that matters is that it's a nutritious liquid.