Here's the problem:
There is no credo that one must sign to be quiverfull. A person who rejects birthcontrol on the basis of their Christian faith is "quiverfull". But they may not be Quiverfull with a capital Q, depending on what that means to them. There is no Quiverfull organization to which a person applies for membership.
Originally, the term simply meant viewing children as blessings, and not using contraception. In that sense, the Duggar's certainly are "quiverfull". So am I. They have 19, I have 3.
Like them, in many contexts I refer not to use that term because other people have taken it and made something it is not. When people point fingers and say "You are Quiverfull, you horrible person", they have taken that term and put under it a whole laundry list of stuff--prairie muffin, hypocrite, ultra patriarchal, abusive, anti-education, etc, etc, etc. It is no longer a descriptor of a single belief (rejection of birth control) but an umbrella used to describe a whole mess of beliefs that have nothing to do with birth control. The Quiverful book, and sites like No Longer Quivering have contributed to this change.
Michelle wrote in the book, and elsewhere, that her fertility returns early, and she has to quit nursing due to issues the pregnancies cause with breastfeeding (severe pain and supply issues, iirc). I'm pretty sure that doesn't constitute "weaning early to get pregnant sooner". Because bc is ubiquitous in this culture, it seems hard for people to imagine that a couple could have 19 children without interfering with nature. The Duggar's are unique in their high level of fertility now, but wouldn't have been so 200 years ago. What sets them apart from highly fertile families back then is that all 19 of their children have survived pregnancy and birth and infancy and early childhood. Yet and still, Susannah Wesley (mother of Charles and John, writers of hymns and evangelical revivalists way back when) raised a similar number of children, and was a child of an equally large family. So it happened.