I suspect that the two part bit is to standardize things. Spire is in the first research block after Scout in the new tech tree, and is opened automatically after Scout is unlocked in Chapter 3 in the old one.
Also there is a small difference in provinces needed to open new chapters. For chapter 4 the old minimum was 50, and the new one is 45, while for chapter 5 the old minimum was 75, and the new one is 70.
This is why chapter 5 was affected as well.