It's been some time since I studied these things, but I believe the post was trying to illustrate that the geodesic lines in spacetime created by Earth's gravit

It's been some time since I studied these things, but I believe the post was trying to illustrate that the geodesic lines in spacetime created by Earth's gravity field can be visualized as straight lines after a nonlinear change of the coordinates system. [1] To simplify, these are the lines along which particles move when no outside force is exerted on them--again, considering gravity to be a distortion of spacetime and not a force, very much like the well-known metaphor of steel ball on a rubber sheet, which would curve marbles towards the "well" it's created in the sheet. But you're right that once the marbles and the steel ball get to a point where one of the gravitational fields cannot be ignored, this framework becomes less useful. [1] https://en.wikipedia.org/wiki/Geodesic https://en.wikipedia.org/wiki/Geodesic