It may be the "norm," but their point was whether the dual earner approach is optimal. I.e., whether it "should" be the norm. Truth is dual earner started as a way to try and vastly increase the household income. Unfortunately all it did was double the supply of labor and drive down real wages. This isn't a gendered argument. We would all likely be better off if every couple chose one of them to work the 9 to 5, and the other one to manage the home and finances (and pursue side ventures). If this happened we'd hope to see real wages increase for those left in the labor market. To some degree we're already seeing this. Labor participation among males is at its lowest point in decades and still falling. Though it's still early days so real wages haven't yet increased to compensate. Unfortunately this may not happen because corporate America is used to plentiful cheap labor, and as the labor supply diminishes they're pursuing automation strategies. In other words, corporate America has options for adjusting to the ebbs and flows of labor supply which they used to lack.
Even now the dual earner household exists because many people believe it improves their lifestyle. But let's be honest, most people wouldn't want to work if they didn't have to. Sure, in the absence of the need to make a paycheck they'd likely still do productive, valuable activities, but very few like to work for corporate America. Most jobs suck. Most managers suck. Most of the time pay sucks. Having a job isn't (or shouldn't be) special for either gender, it's a necessity to whatever point it takes to keep a roof over your head.
Maybe when the robots take all our jobs we'll finally find a better way of living.