This makes sense, but a lot of the time the "one thing" you are talking about is simply the very last required thing. If you had done things in a different order the "one thing" would be something else entirely. And if you had omitted one of the hundred other things, that daily grind and ton of work, then doing the "one thing" would mean nothing.

In short, I think you are completely right, but it seems the point is that you need to focus on achieving that "massive body of prior work".