From what I cited: "A telescoping tube, similar to the boxy ones used to access airplanes at airports would be needed at the end stations to address the cumulative length change of the tube"

I guess that's because accordion-like flexible sections and carriages moving at 1000 km/h on a tiny air cushion don't go well together, or because such sections, being a lot leakier then welds, would let too much air in.

Right, so the tube changes length. The entire station doesn't move. Also, the trains aren't going 1000 km/h at the station.