That comment struck me as very true. Would Van Gogh really have cared that his paintings have sold for millions after his death? I wonder.
Personally, now that I am "old", I am not sure that the idea of success is what drives me to do art, unless it has to do with the Greek concept of arete, achieving one's best in whatever you do. And to me trying to achieve that excellence is a very personal thing.
I resonate with what Van Gogh wrote, as after I had a little show some years ago I was inspired to paint something new, and I thought "This will sell" -- and I quickly realized it was terrible: dead, with no inspiration or meaning to me. I was so discouraged. It took me some time to get back to being myself and doing what I wanted and needed to do. I have struggled with this always, thinking to please others (such as contests) and getting back to making something I'd really like on my own wall.
Somehow I think this is what Vincent meant as well.