Great eye subsonic! You're right, it's double sheet of 3/4" MDF for the entire baffle. The internal volume of the enclosure remains the same because I used the same dimensions for top, bottom, and sides. Imagine that the entire enclosure just moves back 3/4". All the dimensions/angles are the same. The only difference, like hawkeye mentioned, is the 6.5" drivers have another .75" of baffle. The interior baffle has a .75" radius. Also the alignment between woofers and tweeter remains the same.
I thought about this a lot before getting started. I just could see no reason to not make the entire baffle 1.5". It's stronger, more rigid, and easier to construct this way. And I also have seen several other X-Statik builds with 1 1/2" baffles.