He explains it as if the project only costs him money. However, it also helps build his resime/image, which may contribute to him being able to ask such a high rate ($250/hour).
Exactly. Even at 20 hours a week you're looking $20k/month. As a freelancer that is a great rate with little overhead. There is no doubt that being the author of Hapi is a major reason why he can charge that rate. I get being burned out and/or wanting support but not recognizing the correlation just comes across as entitled.
Opportunity cost is still a cost. If he was not working on the project he could be billing clients for that time
Yes, but the point being is that if he wasn't the core maintainer would he still get that same rate?