I certainly won't disagree that the rejection was a mistake (since admitting that mistake is the whole idea of the post).
However, it is worth noting that when we rejected Dan, we rejected a skilled developer. We did not reject a (skilled+proven) developer as you asserted.
Our mistake was in failing to see past Dan's relative inexperience in the areas that mattered to us. We knew he was skilled, but we were unsure how well his raw talent would translate to our product needs at the time. In other words, he was not "proven" in the ways that mattered to us.
Obviously, Dan has now developed enough of the social proof needed to be classified as (skilled+proven), but I think he would be the first to say that his skillset last year was very different than it is now.
Many companies regularly pass on very good people who simply don't fit an immediate defined need. I think this is a common mistake, and one that isn't just made by corporate HR drones and technical recruiters. Part of the goal of this post was drawing attention to how common this mistake is.