According to this, not everyone who knew about the camp was a permanent resident as it quite clearly states that

Supply ships made regular stops at the world.

If everyone who came to the planet was required to stay permanently, the supply runs would have, most likely, ceased after the first handful of runs. After all, who would want to make a supply run from which no one ever returned?

According to this, Shrek was an information dealer. It's likely he did business with the supply ships or with one of their other trading partners and found out about the planet that way.

According to this, not everyone who knew about the camp was a permanent resident as it quite clearly states that...

Supply ships made regular stops at the world.

If everyone who came to the planet was required to stay permanently, the supply runs would have, most likely, ceased after the first handful of runs. After all, who would want to make a supply run from which no one ever returned?

According to this, Shrek was an information dealer. It's likely he did business with the supply ships or with one of their other trading partners and found out about the planet that way.