I have an unusual new theory that these researchers had an agenda.
If the Moai were constructed by a much larger group than the Europeans encountered, that would clear up some of the mystery surrounding the island. But it would also raise more questions. How, for instance, did the population fall so quickly in the few centuries between the statues' construction and first contact with Europeans? One theory is ecocide, which happens when an area is exhausted of its resources faster than it can replenish them.
Evil humans with their evil carbon footprints ...