Really? Based on some quick googling it looks like "middle class" usually encompasses household incomes from about $35k-100k. You think that many people essentially contribute nothing to their 401k, HSA, etc?
That was based on a survey with the question "How much money do you have saved in your savings account?". I doubt most of the people who answered it included their 401k or HSA.
Many work at jobs without those benefits, or take out loans against their retirement accounts. Also, I'm not sure if most calculations offset those amounts with debt.
What's the source for that number? Taken at face value, it sounds unbelievable.