Nowhere does the CBO say that period comparisons are invalid because of dynamism. Appendix A of the full report describes the detailed process required to ensure that data is suitable for that exact purpose. If you want to shoot down the intended meaning, there are better targets than dynamism IMHO. But it's a waste of time: google "CBO Wealth Distribution" and you'll see that the CBO's meaning is well understood.

Since not everybody will understand that much of your quotation is from a partisan attempt to deny wealth disparity, here is what the CBO thought its report was about:

CBO finds that between 1979 and 2007:

For the 1 percent of the population with the highest income, average real after-tax household income grew by 275 percent.
For others in the 20 percent of the population with the highest income, average real after-tax household income grew by 65 percent.
For the 60 percent of the population in the middle of the income scale, the growth in average real after-tax household income was just under 40 percent.
For the 20 percent of the population with the lowest income, the growth in average real after-tax household income was about 18 percent.

Since growth across the period was 62%, middle and lower income people went substantially backwards. These simple concepts are staggering if you plot them on a graph.