Perhaps we should be talking about somatogyral isnstead of somatogravic illusions.
- no - they refer to rotational effects and we are looking primarily at attitude based effects.
Did you see slide 20? The effect of 'head tilt' as they call it - 'nose up' attitude here - which results in a sense of acceleration. Add to that the pressure felt on the back due to the tilt, all of which can cause a sensation of acceleration. I suggest you try to watch a full-motion sim - perhaps the best demonstration would be on the 'landing roll' when the sim adopts an extreme nose-down attitude to produce a deceleration effect where you are pushed forward in your harness..
You cannot just rule out somatogravic illusions in 447. It appears they did not really appreciate the attitude they had, so they could well have felt misleading 'acceleration' effects. I have no idea. I take it you have never experienced 'the leans' or the feeling of pitch induced by rapid acceleration/deceleration without 'normal' visual clues - very powerful illusions and the cause of many accidents..