That is indeed evidence they got there by boat, although before DNA comparison it wasn't clear where they came from. (DNA sequencing has revolutionized determining where people came from and who various groups are related to.) It is not sufficient evidence of repeatable navigational skills. Setting out in a boat and luckily happening upon an island is very different from reliably navigating back and forth.

There are modern examples of people in lifeboats drifting incredible distances in the Pacific, with zero navigation.

People didn't accept that the Wrights had flown, either, until the evidence was indisputable. And rightly so, as it was an extraordinary accomplishment.