The thread referes to fisheye lenses. The 50mm and the 90mm isn't a wide lens by any means.
The FoV from the redcat (250mm) is 5 times the fov from the 50mm. So the calculate Error is the same for both lenses.
EKos/KStars calculation:
50mm on the asi120mm = 330' x 247,5'
250mm on the asi120 = 66' x 49,5'