He doesn't seem to be indicating anyone plans to live there permanently...it sounds like it is intended as a getaway, rather than a home. No mention of building gardens, major construction, or dealing with longterm freshwater, garbage and sewer, etc. In other words, he's building a private campground, not a commune.
In this case it's just shared vacation land which is even easier, and they'll each get their own plot. My family of 6 lived on a quarter acre in a suburb and that always felt like plenty of space.
Also, as noted below, none of us are planning on living there full time-- just visiting over the summer, hopefully all at the same time.
Then I envy your youthful outlook. :)
By the time I was 30 I had had enough of communal living for several lifetimes.
Yep. It was:
http://www.atariarchives.org/deli/cottage_computer_programmi...
So, not counting significant others, ten people sharing a five-acre island. That's 1/2 acre per person except that not all five acres will be flat, buildable land, and you will have to sort out who lives where, without friction.
This works when you're young, before you start thinking about issues like privacy and personal space. By the time you and your friends are 30 years old, the island will likely be a distant and probably bitter memory.
I speak as someone who lived through the 1960s American back-to-the land, communal-living, private-property-is-a-crime phase.