Not sure how a rock carving can be dated. No living matter associated so no Carbon 14 to analyze. Also. “Once part of an island” implies higher sea level. Somebody inform Greta and Al.
Maybe they used microerosion analysis. It’s relatively new.
https://www.researchgate.net/publication/229558469_A_new_method_to_date_petroglyphs
The author dates it when he signs it.