Playing around with the Fat-Line bass patch on the Polysix, here's what I'm understanding in the end. So yes, the amount of Unison voices is 1 and not 6, I also got it wrong interpreting it the first time. But the value 6 for the "Total" voices slot has nothing to do with Polyphony, because both can't be used at the same time, it's one or the other.
This value 6 in the end is the maximum Unison Voices you decide to set, from 1 to 32 and depending of that number you won't be able to assign more voices in the Unison slot.
But in the end yes, it's a 1 voice Unison patch unless you increase the voices, but not related to Polyphony mode.