I really dislike the tone and content of this advice.
I am a solo founder. My first venture (as part of a team) failed - even though we had a product, we had no paying customers and there were other issues. It would have been terrible had someone given advice like this right after that happened - "you're not cut out to be a founder because X or Y." One of the great things about being any kind of founder is learning from mistakes, picking yourself up off the ground, and then creating something that works.
Yes, I think it's the no 1 reason I saw solopreneneur friends fail. Most are just not willing to put the hours in. I see that people really like to dream and make big plans but sitting down consistently after work each day and actually working on all the mundane small things is a necessity many are not willing to do. The idea/business might still fail but without actually doing anything you have lost already.
Entrepreneurship is as glamorous as being a doctor at the emergency room.
But a good salesman can even sell a mediocre product to more people than someone with a good product.
And he'll grab more money to, because he isn't afraid to raise the price