I think they misinterpreted the statictics which they linked. According to a https://www.census.gov/newsroom/stories/single-parent-day.ht... it is 23%, assuming that the share between boys and girls is the same, which is still a lot but not 40%.
Further, some of those 23% are bound to be temporary situations. My initial gut reaction to your link just says "whenever this census was conducted 23% of minors had only one adult in the house."
Not trying to downplay the issue of single motherhood, but the statistics available just seem fuzzy.