Strange, that they call it densely populated, it's quite average for a European country.
https://en.wikipedia.org/wiki/Area_and_population_of_Europea...
I'd say, for it's size, Netherlands is densely populated.
So depending on where you are you can travel for 150 Km and never leave the built-up areas or you can travel for 150 Km and you'll see nothing but grass, cows, the occasional farm and the highway you're on (A6/A7).
The Netherlands is most certainly not quite average as far as population density goes.