What if you had 100 uniques / month with a DAU/MAU of 50%? Insanely successful retention, right? Except it's only 100 uniques! The more important number is the "top of the funnel," which is the ultimate driver of sustainability.
If your point is that you can't draw conclusions from statistically insignificant data, then yes I agree with you. DAU/MAU does not work well when you have only a few visitors. Either way, it looks like the app in question had enough data to draw some conclusions.
aren't retention, engagement, and acquisition equally important? you optimize whichever is your bottleneck.