I dunno about that. Henry specifically told Michael once he leaves on compass heading 325 he wouldnt ever be able to return.
IMO, the electromagnetic field generated a "cloak" over the island. Now that it has been destroyed it released an electromagnetic pulse which will be triangulated back to the island.
Or, maybe not.
Clearly that line is BS... otherwise no one would be able to get there at all... and Desmond got there somehow
If there's a way out, there's a way in.
I think all of the lostaways (except the Dharma employees) were sucked in either on purpose (for children?) or by accident (like flight 815 when Desmond let the system briefly fail). If you notice, all lostaways arrive on something metal (flight 815, the ship the French chick was on) that just so happened to get too close to the island. Well, except the hot air ballon guy, I havent got a good explanation for that one.