The TLDR of that article is that most of the "shrinking middle class" can be explained by changing demographics in the US - i.e. the country is getting older.
Older people, pre-retirement, generally earn more then younger people. As it should be - they've spent their lives growing their net worth and gaining experience.
Most "class warfare" can be explained by this - except that it doesn't get clickbait outrage reaction to say "older, experienced people are wealthier than young people starting their careers".
It's much more exciting to skew charts and graphs to show massive wealth disparity, as if there's a sinister cabal of wealthy folks that for some reason don't want others to succeed.
These charts are utterly meaningless if they aren't broken down by age group. You're putting someone who has 30+ years of career growth against someone out of high school and then screaming about wealth disparity.
The article itself even points this out - they say most of the loss of money in middle income was made up for in the 45-65 year old bracket. After 65 (surprise) when folks are retired, this drops off.
The issue isn't some sort of "class warfare" boogeyman. The issue is that we've made it damn near impossible to have a large family these days, and our population demographics are reflecting that, along with income.
Also note: the article doesn't talk about absolute numbers or wealth. It's all percentile stuff. It's easy to play statistics tricks with that.