They have already found evidence on the island that there were Americans on that island. The artifacts suggest that they belonged to either her or her navigator. A knife was found, for example.
It's come in bits and pieces over several years, and from the same general site on the island (except for the aluminum). From memory: part of an American pocket knife of the brand/model listed on Earhart's manifest; the American heel to an woman's blucher (flat-heeled shoe), Earhart's era and approximately Earhart's size; the broken bottom of an American hand lotion bottle of a type used by Earhart, of her era; part of a cosmetics compact of the type used by Earhart, or the same era; a Sextant box of the type used by Fred Noonan, of the right era and style based on serial number; and a few other items, such as aircraft aluminum used by natives (it was once inhabitated) on the other side of the island with a rivet pattern similar to that of an Electra.
Nikumaroro is a tiny island in the middle of nowhere. Whoever she was, an American woman was appears to have been stranded there during Earhart's era.