>I don't understand why Twitter chose to remove it for "hacked materials" rather than saying it was fake.

Probably because they have no proof it is fake.

One of Biden's campaign surragates, Jenna Arnold, said "I don’t think anybody is saying they are inauthentic or not." It seems like the Biden campaign is not even denying it.

If they removed this for being fake and it turns out to be accurate they would be called bias for not removing fake things about Trump.