I think the milkshake is a step in the right direction. Many other popular icons (envelope for e-mail, key for authentication, gear for changing inner working aka settings) borrow from a non-digital analogy, and I think that's what the author was trying to represent with the milkshake. The milkshake unfortunately does not quite meet the analogy. When you share content, you are not sharing something that is limited physically (like a toy, food, or drink). Rather you are spreading the word about a piece of information. Two more accurate representation that I can think of would be one person whispering into another's ear (for DM), or a person on a podium like he/she is giving a speech (for broadcast).