The author already had a successful exit, so they have their money. Now they want to make a difference.
As far as making a difference, I think calculating that is pretty complicated. Elon Musk had two big exits before he did more fulfilling work with SpaceX and Tesla. Some people want to make that level of massive impact. If that's the case, it probably makes sense to sell, take the money and tick off another exit on your resume, and start your next journey. But if someone is happy with making a smaller difference (I know I certainly would be!), then this might be your big chance to, so selling would not be personally fulfilling at all, even if it is financially fulfilling. There's no guarantee you're ever going to make $6 million from your company, but there is also no guarantee you're ever going to start a company that allows you to have the kind of impact you're having.
While I hate the idea of selling early, there is the practical matter that a small exit allows for a lifetime of future startups. If Alex didn't already have money, I think the best answer would have been to sell. You only get one life.