Hi JD-EE,
He was leaning back. That felt like acceleration to him.
I disagree.
He would know that with a nose up attitude around 15 degs that the sensation on his back felt normal. Most of the time he held around 15 degs pitch - they didn't exceed 20 degs of pitch at any time. If he was confused about acceleration and pitch attitude he would have pitched up even higher.
Pilots recognise the difference between horizontal accelerations felt on take off and the sensation at some nose up attitude - by using the PFD.
This crew simply used the wrong pitch attitude.
I think he's used the pitch attitude he's associated with TOGA power. OK at low level - but completely inappropriate at cruise FLs and they never recognised the fact they were stalled.