The arXiv preprint (v1, June 2026) by Jiaze Li models lightsail dynamics under relativistic conditions by deriving equations of motion from radiation pressure on incident, specularly reflected, and diffusely scattered photons. Unlike prior high-level Starshot concept papers, this work numerically integrates the velocity-dependent thrust terms and identifies a critical velocity where diffuse scattering switches from thrust to drag while net force remains positive. Methodology relies entirely on analytical radiation dynamics and numerical ODE solving; no physical sample or hardware testing is involved. This reveals that 80-90% of terminal velocity accumulates in the first fraction of the acceleration phase due to rapid efficiency drop-off, an insight missed by non-relativistic treatments. Related peer-reviewed analysis in 'Relativistic Lightsails' (Acta Astronautica, 2023) and the Breakthrough Starshot feasibility study (2016) similarly note Doppler losses but omit the diffuse-scattering sign-flip threshold identified here. Limitations include idealized sail reflectivity assumptions and neglect of thermal ablation or beam jitter, common gaps in preprint modeling. The work underscores that mission designs must prioritize ultra-short, high-intensity boost phases rather than sustained acceleration.