Just because you can pick your own hours/work schedule, that doesn’t make you an independent contractor. At least it’s not determinative in the US under federal law nor any state that I am aware, but maybe that is the Dutch law and the court just got it wrong here
When I was freelancing, clients could not tell me at what time I had to be in the office, when I could live, or on which days I had to go, to avoid me being considered an employee.
To determine if they are contractors we look for ways the user/driver relationship is different from other businesses using the employee model.
If no other businesses allow employees to set their number of hours with such flexibility then that is evidence the relationship doesn't fall under the employee model.
This need not be all or nothing but it is (obviously) a factor.