(direct link to the message in question)
I think the point that @dieterdreist was trying to make in flagging that post is to point out that it does not in any way answer the question of “how to celebrate OSM’s 20th birthday”. In fact, it links to a totally separate initiative to grow OSMF’s membership. Which, if we’re being pedantic about it, is not all that different from my linking OSM’s 20th birthday to a totally separate initiative about 
emoji. Really, Arnalie’s serious post was less on-topic than my silly one since I made a direct link to “a way to celebrate.”
If Martin’s point is that, if you’re so strict about moderating on-topic-ness, you end up with absurd results that stifle discussion, then I agree with him.
Actually, I’m surprised that my post with a poem, in response to @ImreSamu’s suggestion as such, survived the on-topic test 