Actually the Prince did succeed in his plan: he absolutely wanted an as private as possible party. And indeed a whole bunch of royals, nobles and other guests had the time of their life, even having a relaxed sports and games competition with each other, larded with a lunch in one of the hunting lodges on the Crown Estate. On Friday there was a private program, a banquet and grand royal fireworks in the palace park. They were barely disturbed by public or media. This must have been absolutely great for all the guests.
I'm sure that it was not the Prince's initial idea to 'snub' the public. It was his initial idea to create a careless and "free-to-breathe-because-there-is-no-media-around athmosphere" on his birthday party. And he did succeed.
The interested public finally did got nice videos on the news and lots of pictures in the newspapers, the magazines and on internet. I have not heard anyone so far in the Netherlands complainig that it was so discrete. There was -as always- nagging about the costs, but as this is a private event all costs are for the Prince, except the security which is a responsibility of the State. Since that became clear the nagging ended (except for the occasional die-hard here and there of course).
Prince Albert of Monaco is a head of state, by the way. And he has never had any substantial relationship with the Prince of Orange and/or Princess Máxima. In contrary to Crown Prince Frederik, or Crown Princess Victoria, or the Eearl of Wessex, he has never been private guests on the princely private estate.