Thank you for your insights. I've been thinking a little more about it today. I think that when I come across something that will take less than two minutes, I have in essence "captured it" by looking at it thinking that maybe I need to do something about it. Then I realize that, yes, there is an action I want to do with it, and yes, it will take less than 2 minutes. I suppose I am applying the workflow process in my mind without realizing it, but it happens in an instant.
You might even say that "unplanned work" has first passed the workflow processing stage if you choose to act on the incoming work. If your boss asks you to review a paragraph on the spot at your desk, and you choose to do it, you have processed the request and decided that 1) there is an action and that 2) you want to do it now. No need to put it on your actions list because you are doing it, but it has in a way made it through the processing stage.
Sometimes I feel there is a little bit of disjointedness in the steps and models that could be conveyed more cohesively in the book. The three-fold nature of work sometimes feels like it's own special model, but it's nothing more than applying the workflow diagram in different ways. Defining your work is just capturing, processing, and organizing results--with review to keep them current. Doing pre-defined work is nothing more than doing things from your lists or calendars. Doing unplanned work is nothing more than just processing on-the-fly and then skipping straight to the doing. I think when people read the five steps on the website, it can easily be misunderstood as a linear dependency. But sometimes processing can happen without capturing (boss asks you for review now), doing can follow an on-the-fly clarifying/processing moment. Maybe the "three-fold nature of work" could be eliminated as a model if the "do-it" part of the workflow simply included a caveat of "do it now if less than two-minutes, or you can also just skip the organizing step and do it-- even if it takes longer than two minutes". The five principles seem to be connected like a spider web, rather than a linear string, and it seems you can often skip from one to another.