Even in that article with its highly click-baity and wrong headline (Spiegel Online has not been "reputable" for long time) they say that 18 months is merely the period after which they want to evaluate their 5-year program.

The interview clearly says that the rule applies to "the first 6 months of recruitment", which in my understanding of the process means "after the job ad is posted".

Btw, imho Dutch people have a tendency for "certainly good enough" solutions because it keeps things moving (instead of perfectionist stagnation whenever confronted with complexity like us germans often do). This 6-month period of only looking at female applicants during recruitment looks like one of those. Nobody says that this will stay the rule forever. Also consider that they will get very few female applicants anyway, so there is still a lot of room for men.