There is probably someone who can answer this better but from sources I understood that under the Wet op het Nederlanderschap en ingezetenschap (law on the Dutch nationality and citizenship) it was stated that you would lose your Dutch nationality when you gained a different one.

Margriet would be born on Canadian territory and therefore become Canadian. Because of this she couldn't become Dutch.

I found an old Polygoon from when Margriet was born. In it the presenter says that the Canadian government temporarily declared the hospital in which Margriet was born not Canada so she could become Dutch.

Also I found a copy of the law online. Article 7 is about when you would lose your Dutch citizenship.