The point is not about whether a task ends up being less than an hour. That happens all the time, as you say.
The point is that, when beginning the task, it is entirely unknowable whether it's definitely going to fit within an hour or not. Any of the 3 examples you gave — updating a test, fixing some automation, or fixing a bug — are things that can easily blossom into all-day (or all-week) tasks.
And so, with 60 minutes and 3 tasks that might be doable within the time, it becomes that much more difficult to justify investing the time to get into flow exactly because of that unknown ROI. Plus, you need N minutes to get into flow and M minutes on the other side to prep for the meeting, so you only have 60 - N - M minutes to invest anyway.
There's a guarantee of an interruption to flow within the hour, but there's no guarantee the probably-small-but-possibly-enormous task will be done by the time that interruption arrives.
This seems to imply that the length of all tasks is unknowable until undertaken, which isn't really true. For those that cannot be estimated ahead of time, I agree with you, but also refer back to my point that all programming tasks do not fit that criteria.
The point is not about whether a task ends up being less than an hour. That happens all the time, as you say.
The point is that, when beginning the task, it is entirely unknowable whether it's definitely going to fit within an hour or not. Any of the 3 examples you gave — updating a test, fixing some automation, or fixing a bug — are things that can easily blossom into all-day (or all-week) tasks.
And so, with 60 minutes and 3 tasks that might be doable within the time, it becomes that much more difficult to justify investing the time to get into flow exactly because of that unknown ROI. Plus, you need N minutes to get into flow and M minutes on the other side to prep for the meeting, so you only have 60 - N - M minutes to invest anyway.
There's a guarantee of an interruption to flow within the hour, but there's no guarantee the probably-small-but-possibly-enormous task will be done by the time that interruption arrives.