"High birthrates and decline in defections spur growth"
While interesting, this is not "unexpected", nor is a "new" story. It shows up in the "news" every now and then. For instance, two years ago there was a story about the "Amish Baby Boom".
However you look at it, the lesson is clear in observing both the Amish and the Shakers. "Church growth" by God's appointed means works, while despising children leads to decline or extinction.
You don't have to be anabaptist and reject "hochmut" to increase your numbers and have a high retention rate!
All that is needed is for us to allow God to cause the increase according to his gracious will. Our vocation is to diligently and lovingly catechize the children he blesses us with, which is the primary calling in this life for parents.
The story also mentions "falling defection rates among adults" as one of the reasons for Amish growth. I would say that while church discipline is perhaps important on an individual basis, I don't really think it is one of the major reason for Amish growth, and neither would it be in faithful Lutheran communities.
However, not allowing sinful lifestyles to appear acceptable is certainly important. The openly unrepentant (and undisciplined) contraceptive mindset that exists today is a primary example. But, again, this is mostly a vocational matter of faithful preaching and teaching by pastors and catechizing by patents. At the same time, church discipline must not be neglected.
In Christian churches today, there is a very high "defection rate" from the teachings of the church - a defection of members that isn't even reflected in membership roles because they are allowed to remain as communicant members. Unfortunately, they're not only tolerated. These unrepentant sinners are put high on pedestals and even appointed as pastors. Like soaking yeast in warm water, these deviant lifestyles and heresies are celebrated so as to even more rapidly leaven the whole lump.
Meanwhile, we are falsely accused as haters, legalists, and pharisees for holding fast to the truth of God's Word.