Thank you Baptiste, I see the oddity in the phrasing, though I also understand Tim's point that the sentence is trying to say "do not use append here even if you are tempted to do so".
Accepting since this could be better phrased, my proposal (as a starting point):
Using list.insert(), as shown above, ensures that the progress bar handler is placed at the beginning of the list. Since upload handlers are executed in order, this placement guarantees that the progress bar handler runs before the default handlers, allowing it to track progress across the entire upload.