We only have the self-confessed hearsay of a royalty reporter for boulevard magazines to go on for this supposed gathering of royals. He may be correct but the article itself isn't very convincing.
Huis ten Bosch is small & does not have many guestrooms it is difficult to have them stay there, esp. if Argentinian family is sleeping in the palace too. That means that royal guests would have had to travel around in The Hague, either from their hotels or from Noordeinde Palace. It would be surprising if this was possible without any pictures showing up on social media.
The 16th birthday is not a special occasion in The Netherlands nor is it one in Argentina where apparently the 15th birthday is celebrated with more attention. I do not know anybody who has celebrated it in a way that is different from a 15th or 17th birthday.
I suppose any dinner at the palace can be described as 'chic', considering the location of the dinner is a palace.
The Queen herself said that there would not be a special kind of celebration as Amalia is too busy with school work. Why should we not believe her over the hearsay of an article which does not even a hint why we should think it credible and which claims in the very first sentence that 'it is not known' who were present.