To be clear, Perfect Day doesn't make "milk" like plant-based milks (think almond "milk", oat "milk", etc). They bioengineered some yeast to grow whey protein directly. The milk they make (made?) probably wouldn't be considered "milk" in the strict sense (they had to get the fat and sugars from plants), but there's really not a good reason to distinguish between "whey protein from cows" and "whey protein from yeast" when it's the same stuff.