The article is not about the number of divorces going down, it is about the rate at which it happens among married couples. It never included those who were not at one point united in marriage.
from the article
"Divorces hit a historical high point in 1979, when 22.6 marriages out of every 1,000 broke up, according to researchers at the National Center for Family and Marriage Research at Bowling Green University.
By 2017, the rate had dropped to 16.1 divorces for every 1,000 marriages. Thatâ€™s a decrease of 29% from the high point and the lowest the divorce rate has been in 40 years".
if you Google the headline you will find access to the article