I've read that the development of psychosis symptoms in general tends to begin with auditory hallucinations (and thought disorder, scattered thinking etc). I think auditory is overall the more common hallucination, probably considered less complex and a lesser severity of break with reality.

So I would guess that because hallucinations are not very common (?) for bipolar and that it tends to happen at the extremes, and it's not necessary for diagnosis, that they're implying that bipolar experiences milder forms of psychosis than the schizophrenic spectrum. So you could start with auditory but it's less likely to increase further into more complex visual hallucinations.

Although, I've had mild visual hallucinations without having anything auditory. So who knows. Maybe the moral of the story is they don't know much about how this all works.