Indeed there are always different numerical outcomes when you change your methods and definitions, so it is possible that by their definition things aren't as bad as the "other numbers" show.
However if you look at the national minimum wage of $7.25/hr, compare that to inflation or productivity, you'll see that the lowest earners are worse now than they have been for decades.
Ironically, they want to focus on "Medicare, Medicaid, food stamps and some 100 other government transfer payments". Those services aren't needed by people who earn enough to actually live on. The reason those services have increased is because the people who need them are not being paid reasonably. This is also why the top earners (particularly the top class who "earn" via capital gains instead of "income") have seen their wealth balloon over the same period that is being debated about.
The authors can choose any definition they wish, but at the end of the day, there absolutely is a vast and growing disparity between the lifestyles of the <50% and those of the >10%.
When we reach a point where one individual theoretically has $200 billion (not liquid of course, but plenty of that is), and there are still children without food, there is something wrong. What is considered fair will vary from person to person, but there are limits.