Likewise for Catawiki. Their listed salaries were significantly higher than what I think is typical here, but I never saw anything over 100k.
The #1 challenge is that most salary ranges for advertised positions are not listed. If you are talking to a recruiter, the recruiter can usually give a range, but still I've never heard these numbers. I get approached weekly for "senior" positions that are in the 50-60 range.
Frankly, I don't believe the high salaries he talks about are numerous. There may be a few, but it would be shocking to find out that 150+k is typical in the examples he mentions.
In the US, particularly west coast and NYC (finance), 200+k is not surprising. 600k? I seriously doubt that is common, unless you're working in a hedge fund and getting 300k bonuses based on company performance.
Generally speaking, the Dutch workplace doesn't seem to hold developers in the esteem that they hold managers. Maybe that's reasonable, but it certainly puts the Netherlands at a competitive disadvantage. The company I work for would like to hire 5 Rails devs, but they cannot find anyone with the desired experience at the offered pay.