I suspect that the name Turtle Island is a recent innovation masquerading as an ancient name rather than something authentically predating the colonial era. If Turtle Island refers to North America only, that would require (in order for "Island" to be accurate) North America to be separated from South America by seawater, which was not the case prior to the Panama Canal (built during the colonial era). But if Turtle Island refers to North and South America combined, there is no indication that any pre-colonial travellers journeyed the entire circumference of the landmass, which is what would be required to ascertain that it is indeed an island. On these grounds I rarely use this name.