I believe the point is that the $900k in ordinary non-retirement savings is being used to create more wealth by investing in businesses. The widening is made worse because the rich have the means to get richer.
I have some pension funds and some other investments. All of these are composed of much the same mix of assets -- mostly equities, hence "being used to create more wealth by investing in businesses". The only differences are that the pension funds (1) have some tax advantages, in exchange for which (2) I have to wait a while before I get to enjoy them.
The tax advantages aren't huge, so #1 doesn't make that big a difference to me. I'm not a big spender and don't need the money urgently, so #2 doesn't make that big a difference to me. So in practice my situation is almost exactly the same as it would be if I shifted all my pension funds into some non-retirement-focused investment vehicle, or if I shifted all my non-pension investments into pension funds. But Ruccio would consider me much better off in the first case than the second.
Obviously it doesn't matter a bit what Ruccio thinks of me personally. But if the idea is to measure inequality, then the thing we're measuring inequality of ideally shouldn't be wildly sensitive to changes that have minimal effect on a person's actual quality of life, likely future wealth, etc. I think a net wealth measure that excludes all housing and pension wealth (like Ruccio's) fails that test, whereas a more normal reckoning of net wealth passes it.
(In my ideal world, the measure of wealth we'd use for this sort of calculation would include all assets and liabilities. Not just stuff you currently own or owe, but a bunch of other things that make a huge difference and almost always get neglected in these analyses. Future earning potential: a newly minted Harvard grad just starting a new job at a hedge fund may have negative net worth because of student loan debt, but s/he sure as hell isn't poor in any useful sense. Access to government benefits: some people have negligible assets and no earning power but receive a pension, disability allowance, etc., which makes their situation less wretched. Likely inheritance: children of wealthy families may be much better off than they look if you consider only what they already own. Future healthcare costs: if you are in the US and seriously ill, you are probably much poorer than your nominal net wealth suggests. Etc. But trying to include all this sort of stuff would make computing wealth distribution statistics much, much harder.)