I think that's a mixup. The mix happens on your stick inputs, it rotates roll and yaw by the camera angle before they reach the PID loop, so there's no attitude estimation involved. Works fine in acro, no accel needed. You can set it from CLI with set fpv_mix_degrees whether ACC is on or off.
You might be thinking of the OSD artificial horizon, which uses the same value and genuinely is dead without the accel.
Still curious what people running a non-zero value actually think of it in the air.