I don't think "campaigning" exclusively means rallies. I understand Biden's war chest is pretty big, and I suspect he has inundated the airwaves and cyberwaves. I don't know it for sure, but I don't think you are suggesting that Biden has not bought ad-time/space in Pa. So I don't think you have a basis to presume Biden isn't campaigning. He's just not holding a lot of rallies. While that is true, it's also quite possible that the average voter (who is not MAGA) finds the rallies during the epidemic to be a very poor choice. In that case, the rallies are actually hurting Trump, not helping. I tend to think more hate Trump than love him, and that his rallies have an equal and opposite effect on both groups.
Those are reasons explaining, in part, why someone not holding large outdoor rallies could beat someone holding multiple outdoor rallies.