A couple of possible reasons the article doesn't consider. The researchers supposedly controlled for similarities extant at the time the couple originally formed, after which the growing-together phenomenon still existed.
They did not control for the "couples who do not become increasingly similar do not stay together" possibility. In other words, instead of long-time association causing growing together, perhaps growing together causes long-time association. Couples whose life trajectories take them in different physical and psychological directions probably are much more likely to separate.
The other factor not specifically dealt with is shared environment. Long-time couples tend to live in the same place, eat mostly the same foods, share recreational interests, travel together, worship (or not) tegether, Perhaps we should be surprised if they did not "grow together."
Birds of a feather not only tend to flock together initially, the absence of continued similarity over time very likely leads to separation, divorce. Hat tip to the other DrC, with whom I've happily lived for 45 years, for the link.
