Thank you for this detailed reply! This agrees with my intuition that the numbers the volunteer told me were not at all realistic. My main goal with writing this post was not to question the effectiveness, but to bring it to the attention of the community that there is value in making this kind of information (such as what you write in this comment) available publicly, instead of the current silence on mega-charities.
Where can I find more information about the tax deductability in the Netherlands? I can't find any information on the main donor lottery page.