Dear Gabriele,
Your inertia formulas are valid for a thin cylinder with a uniformly distributed mass. However, the mass of the OC3-Hywind spar is not uniformly distributed. Instead the mass is likely concentrated closer to the center of mass (due to ballasting), thus lowering the pitch/roll inertias. However, the exact details of the mass distribution have not been made publicly available for the OC3-Hywind spar.
Best regards,