Rasmazzer:
The tutorial is done with ArchiCAD 14 or earlier, you can tell by looking at the Geometry Methods for the Roof Tool (at 0:25) and seeing the Dome option, which was eliminated in ArchiCAD 15 (also, the video was posted in 2008). It is a bit confusing, as the PolyRoof Geometry Method icon in 14 is the same as the Multi-plane Roof icon in 15. So the Roof is composed of single plane roofs, which can be connected as shown using Command-Click (on Mac) or Control-Click (On Windows); in the AC 15 Help files see: Virtual Building > Construction Elements > Roofs > Editing Roofs >
Intersect Single-plane Roofs.
I was able to create the roof you show by using the Multi-plane Geometry Method with the Rectangular Hip/Gable Construction Method and stretching the perimeter back into the roof.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David