Yes you’re right of course; the screw 274 (474 is the longer for kickstand) at the bottom tube alone is too weak to preserve the excenter from turning under load.
After loosening of that bolt there are just two cases possible; either the excenter can be moved or not (that means the groove nut is tight - or not).
Both cases require the mentioned hook key explicitly.
(To derail the belt with a screwdriver without releasing belt tension is a no-go in my opinion.)
At the manual there is one major failure more - it is nonsense to tell a load for the belt but NOT the amount of deflection. 
Another bad trap is the rear wheel bolt 379; be careful not to overtighten it - but it must be glued!