Actually, that kind of wild extrapolation based on minimal data is typical of Holmes. Doyle could just have Holmes’ deductions turn out to be correct, no matter how preposterous they were.
To be fair to Doyle, there were a bunch of weird theories about where Maori came from that were floating about at the time. A lot of Europeans couldn’t accept that an indigenous culture could be smart, accomplished, industrious and brown, so there were all sorts of ideas about how Maori weren’t really Polynesian, but were in fact a lost tribe of Aryans/Phoenicians/Irish/insert acceptable ethnicity.