I pondered that myself. Then reminded myself that this is a nonhuman. There's nothing which says milk gland can't be under or incorporated in a layer of muscles. Whales are able to actively squirt their milk, if I remember correctly, to help deal with the problem of trying to nurse while underwater.
And we don't know for sure that Maenads are mammals. Those might not be breasts.
Yeah, it looks like an oversight. But, y'know, the fact that it would be such an obvious error in otherwise well-researched and well-executed work is exactly what makes me suspect it may have been deliberate.