One reason that we should consider discounting Gatty's conclusion is that we now know stuff that Gatty did not know. Gallagher reports that the sextant box had two different numbers on it and he reports what those numbers were:

3500 and 1542

We do not have documentation for those specific numbers. On the other hand, if you take a look at the second link I posted (https://tighar.org/Projects/Earhart/Archives/Research/Bullet...), you'll see they have an actual picture of a Brandis sextant box with the numbers 3987 and 1542 on it. They have a couple of tables with other sextants that they have identified with similar numbers, including:

  Brandis 3511  N.O. 1585
  Brandis 3527  N.O. 1599
  Brandis 3987  N.O. 1584

That doesn't conclusively identify 3500:1542 as a Brandis sextant with Naval Observatory number 1584, but it does look mighty suspicious.

If that is indeed the case, then the sextant box the islanders discovered definitely dates to 1919 or later, as does the skeleton that was found near it.

Even if we accept this conclusion, it does not constitute certain proof that Earhart landed on the island. It would seem to provide some solid support for the idea though.

If we combine this with other independent lines of evidence (radio reception reports, interviews with people who were on the island in the early 1940s, etc.) you can make a stronger argument.