I think the big disconnect here is in viewing veganism as a strictly biological lifestyle choice, vs. one that is rooted in morals and a particular world view.
If you're applying a strictly biological definition, then I can see how it could be argued that it is not vegan. Technically, biologically, it is an animal fluid no less than cow's milk or goat's milk.
From what I can see, though, most vegans view their choice as a moral one, not a biological one. They're pleased when they find biological evidence for the superiority of their choices, but that doesn't mean they're doing it for scientific reasons. From that perspective, yes, breastmilk is utterly vegan: it is the one thing you can feed a baby that does not impinge on anyone or anything other than the mother, who's giving freely and lovingly by choice. There's no question about little mice squashed in the fields under tractors, no moral issues with transportation and fuel usage or genetically engineered soybeans or supporting formula companies with their unethical marketing practices or supporting companies that treat their employees like slaves, etc. etc.
Although I'm not a vegan, it seems to me that most vegans come primarily from that second camp, the moral one. I think the people who view their diets entirely scientifically and have no interest in the moral issues are an abberation in that community. Ergo, yes, breastmilk is entirely vegan.
I guess what it comes down to is, we shouldn't be asking vegans if breastmilk is vegan or not. We should be asking them if their diet is primarily a moral choice or a scientific choice. Telling vegans, who've made their choice for moral reasons, that breastmilk is not vegan is like telling a Jew that peanut butter is not kosher for Passover because a peanut is technically a legume. That person would look at you like you're crazy and ask you why you're trying to make up your own rules for her religion.