The obliquity between a planet's orbital axis and its host star's spin axis provides crucial insights into planetary formation and migration. Planets with scaled semi-major axes (a/R) large enough to be unaffected by tidal alterations (tidally detached), offer a unique opportunity to study the original obliquity in which the system formed. We therefore observed TOI-1710 b (a/R~36) in-transit using HARPS-N + GIANO-B, collecting high-precision radial velocities to measure the Rossiter-McLaughlin (RM) effect. Spectral analysis of the H{alpha} and HeI triple lines was also pursued to evaluate atmospheric photoevaporation. Using our knowledge of the star rotation period (21.5+/-0.2d), we estimated a true obliquity of psi=149^+11^_-10_deg, which indicates a retrograde motion and places TOI-1710 b among the most misaligned systems -- and the only one known orbiting a cool star in retrograde motion. The strong misalignment favours a high-eccentricity migration (HEM) origin for this low-density super-Neptune planet in the savanna region, challenging previous findings that claimed a minor role of HEM in this period-radius(-density) domain. Moreover, the strong misalignment and lack of a detected close stellar companion suggests a purely planetary post-migration misalignment, likely due to planet-planet scattering followed by planet-planet Kozai-Lidov oscillations and tidal circularisation.