They've only tried 3 positions (<90 deg, 90 deg and 135 deg), and in any case they aren't working with an explanatory theory, so how on earth are they entitled

They've only tried 3 positions (<90 deg, 90 deg and 135 deg), and in any case they aren't working with an explanatory theory, so how on earth are they entitled to claim that 135 deg is optimal? However I believe the optimal position is slightly reclined, for two reasons: (1) only then is there enough passive friction between the chair's back and the sitter's back to provide lateral support, (2) our spines evolved while we were walking and running around in Africa, looking mostly not at the horizon but at the ground several metres ahead, checking for snakes, pitfalls, stones, etc). thus our most natural viewing angle is less than 90 deg wrt to the spine