IMU saturation is an expected condition, especially in this type of extreme real-world environment. It's absolutely normal to temporarily saturate IMU sensors on drones during hard landings for example and saturation values are generally clear in the documentation. A large part of the difficulty in position and attitude estimators is in rejecting glitching or erroneous data.
From the description (specifically that the duration of the saturation was an issue), it would sound like the position estimator (most likely a kalman filter) did not reject or distrust saturation values properly and converged on a bad solution. It appears that any strategy in place to (for example) reset the estimator state if it diverged from more reliable sensors (such as the radar altimeter), failed or were not included. It would also appear that the case where the IMU saturated during descent for over 1 second was not properly tested, as they were able to reproduce the issue in simulation.
This must be disappointing for the ESA but at least it was found now 'in beta' rather than in the relatively more important next lander.