Earlier quoted context omitted.
Straight from the source: https://www.belastingdienst.nl/wps/wcm/connect/en/income-in-... Summary: https://en.wikipedia.org/wiki/Taxation_in_the_Netherlands#Bo...
This looks like capital gains to me. Taxation on income from wealth, not wealth itself. https://www.belastingdienst.nl/wps/wcm/connect/en/income-in-...
The article you linked to refers to a recent court ruling against that system. (A bunched of pissed citizens sued the government because interest saving rates have been 0% or even negative for many years now.) The government is now scrambling to update the tax code, but AFAIK no change has been implemented yet. Last year's net wealth was still taxed at assumed (rather than realized) investment returns in this years income tax assesment.