I don’t think it was an absentee ballot. I think it was early voting, which Illinois allows. The wife and I did the same thing here in Granite City (Southern Illinois). And for early voting in Illinois, you have to show a photo id. I recall the news saying that BO showed one, and joked about his hair being grayer. I think he was passing through.
Even if he was “passing thru” that was still expensive. He was trying to encourage early voting.