But that article says the same thing, since the definition of middle class used by Brookings is from $37K to $144K income, i.e. the vast majority of the “middle class” whose marriage rates have declined are in the lower two groups in the chart I posted.
I don’t like the religious framing of the Deseret News article either, but the fact that Brookings are writing about this subject (and portraying it as a problem) does tend to support the idea that the declining marriage rate amongst those with “middle” (or “lower”) incomes is becoming an issue of more concern across the political spectrum.