Not buying the $1 million. Sure, there may be rare cases that can be made, but the article makes it sound like a slam-dunk "average".
I can believe it. Say a woman is out of the workforce for 15 years, foregoing a $30,000 salary, then returns to work for 20 years but her salary is $10,000 less than it would otherwise have due to her lesser experience. That's $650,000 already, and assuming a halfway decent interest rate you can easily get to $1 million. Of course you could argue with any of those assumptions, but it seems to be in the right ballpark.