That's a great analogy!
On the one hand, the context is different. An affordance, as you point out, signals how we should use an item. In a way, it suggests a meaning. A kanban's meaning is typically defined as the process (largely) is known.
In the story about Milk Kanban, the index card is self-explanatory, but that's just an extra bit, to make the process more bullet-proof.
On the other hand, if we generalize both examples, we land in a place where the design (of things, of processes) should communicate how to act. I either interact with an object or I do my action within a process.
BTW: The Design of Everyday Things is another great reading recommendation in this thread.