There is no contradiction in those statements at all. The Polynesians who settled New Zealand became the Māori - they are the same people - and the phrase 'the Polynesian ancestors of the Māori' is a common one in NZ scientific literature. The point is that there was no one in New Zealand when the ancestors of the Māori arrived. I like the phrase 'Polynesian ancestors of the Māori', but maybe it is confusing to those who haven't heard it before.