My point is that the "duration of the Disengage action" is irrelevant and has no meaning or value. This is supported by the fact that there is zero language in the PHB about the "duration of an action", because in my opinion it simply does not matter. The only thing that matters is the order of the elements that make up your turn. The Disengage action, like other actions, is a discrete element in the ordered list. You process these elements in order, one at a time. Once you've processed the Disengage element, you have taken the action and any event that has that as a trigger can now be added to the ordered list.
1) Move
2) Disengage
3) Move
The movement element (3) does not provoke OAs. The movement element (1) does provoke OAs. Why do you care how long (2) took? We simply resolve each entry in the list in order, and apply the effects of the entry as needed. So, movement might be moving your miniature on the map. Disengage means applying the temporary "your movement doesn't provoke OAs" buff, similar to the buff you'd get from the Shield spell for example. Any movement that happens after this element is resolved does not provoke an OA, nothing more, nothing less. The "duration" of the Disengage action itself has no relevance to this, in my opinion -- it's simple one of the discrete elements on your turn that gets processed in order.