My OB would disagree with you regarding it being an exception and I have to be honest in that I figure she has 30 years of experience and likely is a bit of an expert there

This isn't really the discussion at hand IMO as it is not a reliable means of birth control and I don't think it matters when she weans based on how unreliable it is and hasn't she already stated she has gotten pg while nursing? My understanding is that she has stated either in a book or online that she gets her cycle back by 6-8 weeks pp so it really doesn't matter if she nurses for 3 years or weans at 3 months..she could still get pregnant quite early on and her timing to wean has nothing to do with her having children.
Has your OB said that breastfeeding is *bad* birth control method (as in, it is not nearly as reliable as hormonal methods)? Or did she say that it offers no protection whatsoever?
The latter just does not seem to fit any of the information I can find online. Even planned parenthood (certainly not anti-"artificial" birth control) says that breastfeeding (if it follows very strict guidelines at least for 6 months after birth) is about 98% effective. Better than condoms, diaphragm, etc. Almost as good as hormonal methods. But only that good for 6 months and requires strict following of guidelines about constant breastfeeding. (The strictness of the guidelines probably leads to many more failures than if it were done perfectly. This is probably the reason why it's good advice to consider an easier to use birth control method in addition if you really don't want another child at that time. Similarly NFP if done perfectly does not have such a horrible rate of failure; the issue is that it is difficult to use it 100% correctly. Female condoms, I believe, are also something where there is a large degree of user error which makes it not a very good method overall.)
If the OB said the former than this is compatible with a wide degree of effectiveness levels for breastfeeding past 6 months. Withdrawal for instance is a really unreliable method of birth control, but it is literally better than nothing. (I think there is an 85% chance of pregnancy in a year for users of no birth control, more like 30% for users of withdrawal.) Long-term breastfeeding might be a bad/unreliable method, but even if it's effectiveness is = to withdrawal's, it will still on average in a large group of women lead to longer birth spacing than the Duggar's have.
Say that is the effectiveness is even worse than withdrawal--say 40% get pregnant in a year. Take 100 hunter/gatherer women breastfeeding constantly. 2 unlucky ones will get pregnant within 6 months after birth. Of the other 98 who make it to 6 months postpartum without a pregnancy, 40% of those will get pregnant within the next year, so about 39 women will get pregnant by 18 months after birth (probably spread out throughout those 12 months). That means the majority will NOT be pregnant within 18 months after birth. (In comparison, if breastfeeding offered *no* protection whatsoever, then we would expect 85% of the women to be pregnant within 1 year after the birth. In fact, we KNOW that this is not the case in hunter/gatherer societies or amongst great apes. Births do not occur nearly that close together in "nature.")
Now if Michelle really does routinely getting her period back only 6-8 weeks after birth despite constant breastfeeding then indeed she could probably not expect breastfeeding to give her much protection. She would perhaps be one of the 2 unlucky hunter-gatherers who got pregnant within 6 months of birth despite constant breastfeeding. And in "nature" if she was that unlucky she and her babies would probably end up suffering greatly--with the babies less likely to survive and her dying given so many pregnancies.
In any case, if her full fertility really does return that quickly, I don't think that is representative of most women. Certainly we could not have survived as a species if our ancestors routinely got pregnant only 8 weeks after giving birth. Long birth-spacing without modern medicine/food is important to giving the babies the best chance at survival. How is your average not very well-nourished early human/ape supposed to nourish both a toddler and infant through breastfeeding (as would be necessary in nature without formula/ability to cook foods to make them soft) and a growing fetus plus keep herself alive?