Hi Rudder Rudder Rat (I remember it well!)
"At 2 h 10 min 16, the PNF said "so, we’ve lost the speeds" then "alternate law ...
The airplane’s pitch attitude increased progressively beyond 10 degrees and the plane started to climb. The PF made nose-down control inputs and alternately left and right roll inputs. The vertical speed, which had reached 7,000 ft/min, dropped to 700 ft/min and the roll varied between 12 degrees right and 10 degrees left."
He must have made some unnoticed back pressure input to climb.
I can't help thinking that to pitch up from cruise attitude to "beyond 10°" without increasing thrust would take more than a "light" back pressure.