Take the analogy of an athlete. Any athlete knows that recovery is an important part of training. Too much work and they over train. This can be applied to any area of life but is usually called burn out in business. Burn out happens regardless of how much one enjoys the work.
And with the example of the athlete a portion of their job is to have down time, and not be out doing strenuous physical activities the night before the big game.
When I encountered the situation where my leisure became my job, I simply didn't have anything I wanted to do to fill the free time anymore. Sure I could of done push ups or gone on dates or something else which I personally hate doing, but making myself miserable in my free time isn't my idea of healthy.
I don't qualify continuing to work as leisure time just because you happen to like your work. Work is work, regardless of whether or not you like it.
Also I don't believe that simply increasing the amount of hours you work on a startup will directly impact its success. Cutting the scope is almost always a better way to improve a startup's chances.
80 hours still leaves over half his time for eating, sleeping, the small amounts of physical activity which are realistic, and small amounts of human interaction. It simply does not leave time for leisure or to unwind, but as I said previously if your work is what you would do with your leisure time then that's not an issue, as there is nothing to unwind from.
Now I have only encountered this situation for a brief 3 month project which I wish had never been finished. But this type of dream job type situation does exist, and I don not believe is unhealthy, unless you have some sort of family to maintain also (it doesn't really leave enough time for a healthy family life).