Do atheists believe the bible – at least parts of it – is a real historical document?
I won’t speak for all atheists, but will say definitely parts of the Bible reflect actual history. There appears to have been a Davidic kingdom (even if the details may not be quite as recounted in the texts) and there was certainly a Roman empire and relevant peoples knocking around in 1st century Palestine. I would also agree with scholars like Bart Ehrman that there was likely a historic Jesus, even if the detailed events might not be accurately covered in the limited texts that have come down about him.
The closest counterpart to this point in major religions would be the Koran, which directly recounted the actions of Mohammed in establishing Islam. Most older religions, like Hinduism, carry lots of largely mythological baggage, as seen in Genesis (which borrowed heavily from Mesoptamian creation stories during the Babylonian Captivity–another historical event few in historical community contends didn’t happen, and so ranks as another historical period reflected, with a lot of window dressing, in the received texts).
Fly in the ointment: many, if not most, of the organizers of the March for Science are politically left (scientists have long trended leftward of the general population, and are also less religious), and so politics, including issues of social justice and economic policy, are becoming entangled.