Hi again,
First of all; Welcome to Holland
I haven't got a precise answer on your questions but it's certainly good that you ask here.
Normally all of our dunes, being an active part of our defense against the sea, are restricted areas and are part of our National nature reserves (hence the fences around them).
What I see on the video is that they are standing on top of the dunes, behind the barbed wire fence, which is normally prohibited?
Free areas are normally spoken foot / walking paths, bicycle paths and dedicated (playing) areas.
A reason behind it in a nutshell is that people will damage the grass on top of the dunes, grass dies, sand gets exposed and dunes gets blown away by the wind and after a while we all get wet feet in most parts of the Netherlands

Crossing the fence might get you end up with a fine of I believe 90 euro or something.
To be honest it has been a while ago that I have been living in that region so I can't tell you the specific details you are after.
Hopefully somebody else will comment here but seen the number of other responses I can only assume that the person who made the video entered the top of the dunes illegally.
What should be a possibility is that you start on top of the path between the dunes, do some flying and then land on the beach when you are done.
You are allowed to fly on the beach, just keep common logic ahead of the rest. Stay and act safe, mind the nature and mind the safety of other persons.
And obviously in summer time when the beach is filled with people it will not be so much publicly accepted e.g. just like flying kites on the beach.
Let's see if other people who are more known with this spot will comment.
BR.
Paul