Actually, there isn't enough information to conclude that the “rich simply got richer” — unless you're using a strictly relative measure of “richer”.
According to the legend, the graphic is based on the median income in the metro area. If you lower that median income, by substituting middle income families, with low income families, then a higher-income family that remains, will have a higher relative income — without needing to actually have more income. They will be “relatively” richer — but, not necessarily having more income.
More than anything, it's that the median income gray areas are evaporating and being replaced by green/brick red colors (the extreme opposites of the spectrum).