>But then realized that the coastline paradox must also exist for 3D surfaces (TINs or DEMs or whatnot) so asking “what is the area of a surface on earth” can be a very complex question if dealing with elevation.

StandUpMaths did a great video on this recently, asking whether national land area includes elevation and which nations would gain/lose the most land area if it were/weren't: https://www.youtube.com/watch?v=PtKhbbcc1Rc