Author here (and original poster). That's a great idea. I'm going to write them to see what they think about the origin of the symbol, and will update the post if they respond.

I've always loved the location of that museum by the way - not only beautifully situated facing the water, but also across from a park frequented by sellers of hashish (at least it was when I lived in Lisbon back in 2012). The big iron fence dividing the licit drug museum from the people selling illicit drugs right in front of it has since become my favorite metaphor for the history of drugs.