The question that the judges answered in the ruling was, whether the crime of "theft" was committed by defendant.
That's article 310 of the Dutch penal code, translated (IANAL) as "He who takes a good that belongs wholly of partly to another person, with the intent to appropriate it in an illegal manner, will be, as being guilty of "theft", punished with a prison term of at most 4 years or a monetary fine of the fourth category"
His defense argued that what was 'stolen' could not be seen as 'goods' as defined by article 310. Also important to note is that if something is a 'good' under that article does not necessarily mean that it is also a 'good' for any other law or purpose.
The Dutch highest court does not look at the entire case again, it will only look at certain aspects of it (questions of law) and not of fact.
A lower court already concluded that the defendant was guilty of articles 310 and 312 (theft accompanied by violence --> robbery).
So since the high court concluded that the in-game possessions were goods according to the law, it automatically had to conclude that theft and theft accompanied with violence were committed. And I personally don't see why not, as this is clearly a robbery.
Luckily we don't have mandatory minimum sentences in Holland so judges have a lot of freedom to look at the circumstances when considering the punishments.
Things like how much or what kind of violence was used, how old the defendant was, whether he is a repeat offender, etc.
We don't actually have to twist to meaning of the word "robbery" and call it something else when the minimum punishment seems too high, because some politician who can't stick to his own business forced judges to ignore the circumstances.