I've seen AoPS mention this in a number of different spots (my DS is antsy to get at geometry, so I've taken note). The message seems to be that with the first 13 chapters of Algebra, you have the basic math skills you need to do geometry. However, after the other two Intro classes, you will also have developed a much wider range of problem-solving skills and approaches which will make it much easier to deal with the more complex geometry questions.