I think the point is. given the thermal structure that balances the surface boundary condition and the flux of heat out of the Earth, the deformation results from an elastic equilibrium accounting for the stress from thermal expansion and contraction.

If the thermal conditions were changing sufficiently rapidly, the elastic rebound would make waves. As the resonant frequency of the entire Earth has periods only up to an hour, but global temperature changes have a period of a day or longer, elastic waves are NOT generated. The deformation is "quasi-static", i.e., inertia is not involved, and the motions are not oscillatory (wavelike).

Current understanding is that fluctuations in the temperature boundary conditions at the surface of Earth do not affect the internal dynamics below about 1 meter in depth.