Over time I've ended up not really understanding this advice, but only because of something I accomplished in the past.
One day I just felt like writing something. I hadn't written since elementary school and had never published a piece of writing online before. But one day I just felt like writing something. The idea just would not leave my mind and I failed to get any sleep, instead spending all of my conscious time writing.
Within a week I was more or less finished. It was a short story numbering only a few dozen pages at most. But save for some minor editing it was complete.
And I decided that since I finished something, it was a success. And I also liked what I wrote, legitimately. I don't think it was that great in the context of people with experience, but it was mine, and that was enough for me to declare it a success. To me, I thought it was valuable. To me, I thought it was worth doing.
So if it's true that you must spend a lot of time to make something valuable, then in my view one of two things is true:
1. What I accomplished in only a week was not valuable to me or anyone else and was not worth doing, and I'm just deluding myself.
2. The advice was untrue and you can still create things of value without spending a lot of time - it just depends on what the scale is.
And if there is no way to create something valuable without spending a lot of time on it, then I don't understand why Ludum Dare and NaNoWriMo still exist, unless people believe the things they accomplish during those events are valueless, which doesn't sound right to me.
Since then I have never really felt like writing. I will either sit down and not think of anything for hours, or at most write whatever comes to my mind in a stream of consciousness and then proceed to not look at or edit it ever again. I think this is what people call being trapped by their past successes. I expect to be able to accomplish again what I actually did accomplish a single time in the past, which is not realistic. Not everything I do will turn out as well as that one time. But the fact remains that I called what I accomplished a success despite not spending much time on it at all, and on top of that it was the first thing I had seriously written in over a decade, and I seem to be unable to reconcile that with the practicalities of developing a skill properly.