Names for whole continents was a thing that mostly only started to matter when global exploration, trade, and colonialism started to be things. For most people, continents were too large and/or too close together to meaningfully differentiate from land in general and/or each other. So basically, they didn't used to be called anything, because they weren't conceived of as things.

Turtle Island wasn't the name for North America, it was the name for the whole world - because the whole world those groups who used it knew about was what we know call North America.

Modern usage of Turtle Island for NA is exclusively modern. I'm not saying it's illegitimate at all, just that it's language used by NDN activists today to assert their place on this stolen land.