Possibly some of the poor outcomes are due to married people being more likely to be able to sustain a stable relationship. There are a lot of factors confounded with marriage, and it doesn’t seem like the author controlled for them (this is an article about a book and not a peer reviewed study). She explicitly mentioned that married couples are more likely to have a better household income and that single moms are less likely to have support from outside family members. So I suspect that outcomes might be better for a single parent with the income and family support the same as a married couple, but that situation is rare. Similarly, if you forced two people to get married and have kids when they didn’t want to, those kids probably aren’t going to get the same benefit of growing up in a married household.