In the same paragraph it's also stated that there's a proportional decline in marriages, each declining by ~2 per 1000

The assumption is that there is a "idealized" rate of divorce that would exist if all impediments were removed. The actual divorce rate being close to this is a signal of high economic mobility across the economy and vice versa.

I like it tbh, it's a good big mac index.

I would bet there's also an idealized rate of people finding their endgame that is much lower than the marriage rate and getting closer to that trends with high social mobility.

I interpreted the parent comment in the inverse. A poor economy means people will stay in unhappy situations longer than otherwise.