The thing about gold is, it’s probably quite easy to secretly leave to your heirs.

Yes, I think so too. It's the only worthwhile reason to commit this crime, surely. You'd be relying on the claimants abandoning their claim at some point because surely the statute of limitations doesn't just apply because you were particularly good at hiding something. Realistically, they'd have to sell this to a collector many years later for much less than what they're worth (since they can't be sold on with proper provenance tracking).

It doesn't even seem worth it since the original investors wanted a fraction of the proceeds not all of it. Just seems like a strange choice, but I suppose that's why I'm not an intrepid underwater gold adventurer and this guy is.