This video will show you how to reach the secret out of bounds island in 2 different methods.

1.) Easiest method is rocket grappling, I will show you the best location to do so.
2.) More legit method where you jump from space and land directly onto it.

I didn't see any other guides on how to get to it, only that it existed. I hope this helps!

https://www.youtube.com/watch?v=2ImbEFqA7uU