Nothing delivers perfect horizon in all circumstances (at least not for the costs people are willing to pay for a gimbal even at the high end). Everything will be off now and then. That includes the DJI Zens, the MōVI gimbals, and everything else out there. In a two man setup the camera op adjusts roll to compensate when this happens. In a one man setup you live with it one way or another, touch up roll before a key shot, etc.
The basic problem is that any drift in yaw or tilt is either not noticed or routinely adjusted out in the shooting process. But the horizon drift is so obvious even if it's a few degrees that it garners all of the attention. The encoder on the roll axis can deliver a home point -- that is, a persistent deviation from that originating encoder position is likely due to horizon drift -- so, you could imagine a system that is constantly taking current roll deviation and reverting to some extent to that origination. Not perfect, but nothing is. Encoders can be a help, then, but not a cure-all.
I don't know how Alexmos is using the encoder data in this regard. Would be interested to find out.