Earlier quoted context omitted.
I think it's fairly clear that they mean "the birth rate for people in the 20-35 age range, which happens to coincide with the millenials at the moment."
It's pretty clear what they meant, but humorously even your rewording isn't accurate. A named generation is static, based on the birth dates. The "20-35 age range" is dynamic, anchored to the present time. Also, named generations (like "Millennials", "Gen Y", "Gen X", "Baby Boomers", etc) don't have rigid definitions. Some media groups define it as starting as early as 1976 and others as late as 1984, which is 50% of…
Yes, that's why my phrasing used "at this moment." At this moment, millenials are around an age range of 20-35 years old, give or take a bit depending on the very fact that you point out, there aren't well defined boundaries and the exact age range could vary. I was very careful to include "at this moment" in my phrasing, for the reason that you point out. I didn't bother to hedge about the unclear boundaries of the millenial generation, since that wasn't the topic at hand, but yes, these terms are somewhat vague.
The actual article cited simply discussed the nationwide birthrate at different points in time; it didn't talk about particular generations.
Given that millenials are currently the generation that are of prime child-bearing age, it's pretty clear what the original poster meant "people of child bearing age right now, who happen to be millenials, are having less children than those of child bearing age at any point since the great depression."
The original phrasing was humorously incorrect, I just thought that asking for clarification was a bit much, since it was pretty clear what they meant to say, it was just a funny mistaken phrasing.