It’s a fair critique, but it really doesn’t come close to explaining the 2003–2013 data.
The ratio of 95th percentile household net worth to median household net worth doubled from ~9x to ~18x. That’s not just some artifact of changing family structures. That’s a massive increase in wealth inequality.
Basically, your point is interesting, but in context it’s mostly a distraction. (But I agree that a chart showing individual rather than household wealth would be nice to see.)