Ok, without much thought, my initial reaction when hearing that this was the first time in MLB history that all 15 home teams won was that I was surprised it hadn't happened before. Your post piqued my curiousity, so I did the math behind it, which I should have done before making my above statement.
For simplicity, if every game is taken as a 50-50 proposition, the odds of all 15 home teams winning is 1 in 32,768. Let's say that there are roughly 120 days each season when there is a full schedule of 15 games. Mathematically, this should happen once in about every 273 years. So you were right - it was more likely to be another century before this occurred.
According to Elias, the previous best record by home teams was 12-0 on May 23, 1914.
Good stuff Youk.