You linked an income inequality graph, which is distinct from wealth. If you look at the list on Wikipedia [1] and sort by Wealth Gini (2019) you will find that the Netherlands are #1.
Based on the Global Wealth Book 2018 and 2019 from Credit Suisse, comparing Table 3-1 in both books, it appears that the number of adults in "under 10.000" range increased significantly in 2019. This is likely not a real change, but rather a change of methodology or data sources. As far as I can tell however, this is not detailed in the text. I have directed an email to Credit Suisse on this, as it's a rather interesting piece of data.