The article actually suggests a much more complex relationship than that, doesn't it? It says:
In bivariate regressions, having no engagement
ring and having wedding expenses below $1,000
are each associated with increases in the hazard
of divorce, while spending $8,000 or more on an
engagement ring is associated with a decrease
in the hazard of divorce. This appears to be consistent
with the relationship between wedding
expenses and marriage duration posited by wedding
industry advertising. However, the picture
changes in multivariate regressions. Spending
between $2,000 and $4,000 on an engagement
ring is significantly associated with an increase
in the hazard of divorce among the sample of
men. Specifically, in the sample of men, spending
between $2,000 and $4,000 on an engagement
ring is associated with a 1.3 times greater hazard
of divorce as compared with spending between
$500 and $2,000
It says that having no ring at all gives you an increased risk of divorce, but $2,000 to $4,000 is a higher risk than a cheaper ring - but, if you look at their actual figures, $4000 to $8000 is lower risk than both $2000 to $4000, and $0 to $500.
I know people love this simplistic 'if you spend money on a ring/wedding it means it's not about the marriage for you and you'll get divorced!' narrative (I will put money on this thread attracting a load of smug 'will, we got married for £1.62 and we've been married for 97 years' posts), but their actual findings were a lot more complicated than that - and, it's worth pointing out, barely any of them were statistically significant (for the ring figures only the increased risk at $2000 to $4000 is marked as significant).