>Yes, because history. There are no historical examples where wealth was held by the few and social mobility increased broadly

Uh... The industrial revolution.

You seem to be confusing wealth inequality with income inequality which might be muddying your arguments.

The graphed figure is not wealth, but income. The (great-)grandparent poster is suggesting that as long as people bounce between income levels, i.e., they have some higher paying years and some lower paying years, it has a smoothing effect on the net income inequality.