There isn't anything really specific about early voter turnout that I think is important, but I do think better / larger turnout in these bigger cities, particularly in the blue wall, is meaningful. Certainly in terms of early voting some of it may be cannibalizing as Oblong has suggested, but I doubt that it's entirely explainable in that way. In Detroit's case, maybe some of it is that recent law changes have just made it easier for Detroiters (and Michiganders) to vote, which would naturally just lead to a higher turnout.
Overall, if someone came from the future and told me that Milwaukee, Detroit and Philadelphia each saw their overall turnout increase by 2-3% versus 2020 and asked me who I thought would win Wisconsin, Michigan, and Pennsylvania respectively, it'd be pretty easy to respond "Harris"