1) Clearly the top 50% of the population is doing better, with increasing degrees of better as you go up the income scale. Even within the middle quintile (the middle class) I’m sure the upper half of that population is doing better than the lower half. Also clear, there is not meaningful growth in the lower two quintiles or even, really, the middle quintile. Therefore, both the mean and median income have risen, but you can’t claim that the actual middle class is doing significantly better.
2) Clearly the stock market is hugely significant in the growth of incomes in the upper two quintiles, as someone noted above. That’s fine, but recognize it for what it is, and don’t assume it has any meaning for actual wages that people earn.
3) The data comes from the same survey that generates the monthly unemployment rate figure. The unemployment rate is the buck-toothed stepchild to the actual non-farm payroll number that significantly moves markets on the first Friday of every month. The unemployment rate is viewed with raised eyebrows by bond traders, for what it is worth.