I wasn't suggesting dating it via DNA. My thought was that, should anyone make the accusation that the artifact was from some other wreck, you might be able to prove through DNA that the coral attached to the artifact is closely related to coral found around Niku. I would imagine that coral DNA from Niku is pretty unique, given how isolated the island is. Since coral on the artifact would indicate that it was submerged, and coral DNA might be a little different from one part of the reef to another, you might even be able to tell where on the reef the artifact was submerged. If it is Kanton coral DNA, then the artifact was likely brought from the alleged DC-3 wreck.
Of course I am no scientist and have only a TV viewer's knowledge of DNA testing, so this could all just be dumb talk to someone who knows about this stuff. Maybe you need some different type of coral tissue rather than what we know as coral, which is the skeleton of the creature. I dunno.