according to my understanding, the equation is just approximating the vertical elevation change. In fact, it is still largely representing the change of surfaces along the line of sight. There is no simple way retrieving it from a single InSAR image pair (confirmed by mengdahl here). johngan nicely explained the difference here.
A comparison between the formula and the phase to displacement module is given here - they are nearly the same.
This was discussed this in several topics:
- Difference between vertical and LOS displacement
- Vertical displacement senitnel 1 interpretation
- Vertical and horizontal displacement
- The phase shift originates from horizontal or vertical displacements
Two posts naming formulas: