I think there are a few caveats in the article that might work better if explained. Or maybe I'm just good at rationalizing.
If I read the article and assumed that the income they were talking about was after savings, then the 80-90% concept would be easier to swallow. For example, if you are making $100K/yr, and taking home $40K after taxes and savings, then it would be relatively easy to establish a retirement fund that would allow you to live on 90% of that ($36K/yr). Of course, if you are making $100K and taking home $65K with minimal savings, it's going to be a lot harder to reach that 80-90% level.
When I was working, I was saving about 35% of my after tax income, with my 401K (including matching) and personal savings. But I know plenty of people I worked with who were making basically the same as me, but had a much larger dollar amounton their paystub. It's possible they were manually depositing their savings, but since I've been FIREd for 3 years and they are still working, I doubt it.
The article did say they would need that 80-90% to maintain their current standard of living. Following my definition of income (above), that's probably pretty accurate. The question becomes whether they are saving enough to reach that goal.