I understand that they couldn't use the Escape key, and so having an alternative makes sense, but I'm not sure as a user how I would ever discover the behavior of pressing "shift" three times.
Escape might be more intuitive but it's not more discoverable. Shift is used often when inputting information, and the mentioned visual feedback give this behavior an opportunity to be discovered.
Having said that, regardless of the key the guidelines on using this pattern say that you should explicitly inform the user of the feature before they first encounter it.
