Then on the island they learned another form of writing. This they would have learned from the earlier inhabitants with whom they shared the island for a considerable period. This fits Heyerdahl's Long Ears, Short Ears hypothesis. These are but suggestions. We do not know the ultimate source of the American writing system that is also found on Easter Island.
Solutions are somewhere down the road a ways.
One starts uncertain of the alphabet, or writing system, for not all are alphabets, then of the language, and only then can decipherment begin. And even then there is resistance to there being any writing in America, except for Mexico of course, and even that was not alphabetic. All else is suspect. But it should not be. Evidence is evidence.