Dear Kasia,
The results given in that table (with a pitch natural frequency of 0.034 Hz) are for the full system (RNA + tower + spar + moorings).
I haven’t reviewed your spreadsheet in great detail, but I would guess the big problem is that you are considering the solution as a one-DOF problem with respect to (0,0,0), but the pitch and surge modes are coupled at this location. The spar mostly pitches (uncoupled from surge) about the location where the moorings attach to the spar (0,0,-70). While I haven’t tried this myself, I would assume your one-DOF approach to calculating the pitch natural could work OK if you referenced the the inertia and stiffness about this point. Alternatively, you can perform an eigenanalysis on the linearized full-system matrices (including multiple DOFs), which is what NREL did to generate the published result.
Best regards,