I still read this as a frame of reference with absolute positions. I think the point of GP was that any time travel device you build, calculations/projections

I still read this as a frame of reference with absolute positions. I think the point of GP was that any time travel device you build, calculations/projections you make, will all be built around a frame of reference where the planet IS in a fixed position, as far as you are concerned. Since you don't know any other frame of reference, the machine won't either.