If you click through to the source that Wikipedia is citing [1], you can see that it's counting 1st and 2nd generations, and are indeed categorised as "people with a migrant background" ("met migratieachtergrond").

It also shows that 1.889.704 people (10.74%) have a non-Dutch European background.

Also note that these numbers are about people currently here. The percentages will be different if you look at movements in the past year.

[1] https://opendata.cbs.nl/statline/#/CBS/nl/dataset/37325/tabl...