Not to derail that he's a creep and weirdo for other things...but the Mars island thing is literally just a meetup for 30STM (his band) fans, and the "this is a cult" thing was literally a tagline for those fans from years before (I have shirt that says it) from like 2006.
It's all remarkably tongue in cheek and the Mars island getaway was intentionally supposed to look like the typical view of a cult like we've seen in the 20th century on purpose, but on the site it was just a bunch of music fans and decent food, and campfire and singalongs with the musicians riffing. It was about as wholesome as it gets actually.
Most 30STM fans get a kick out of how the images from that spread as if he really was leading some cult, because it's a massive troll that worked on so many who simply didn't bother to look it up.
TL;DR: Mars Island is literally a one-band like yearly get-together for wealthy fans to go and hang out and chill. It's a marketing stunt and always has been. It's not a cult at all...but the fact that so many memes have run off that people believe that it really is one, is wildly amusing.
~Source I'm a long-time fan of 30STM and know people who attended the first Mars Island.