In the video, the author went to creating a “Family Instance”. As you may know, a roof is built in to the project, so there is no really simple, “Save out somewhere and load in”.
If I hit this same problem and I wanted it to be a roof, I would abandon that last part in the graph of creating a family instance. I would start looking at the process of running the graph in the project environment to create a real Revit roof. Then, I would look at a couple of different packages that you may already have, such as Clockwork and Rhythm which have nodes for Creating a Roof Instance or adding points to a roof. Then, you need to start splitting that surface in Dynamo in to points that you can add to the roof.
All of that is just to get the roof, but then you could also think about the problem in a different way. You could get the bottom of the FamilyInstance (if you still want to go down that method), and then modify the edit sketch of the wall to match the family instance. Maybe something like exporting out the bottom edge of the “Family Instance” roof to .csv files and then importing that line to the sketch. That would be considerably harder and would not recommend, but gives you another option in thinking about the problem differently.
Finally, you could think about moving to the Massing environment. I haven’t messed around there too much with creating items in that environment with Dynamo, but would anticipate(?) it to be possible. Then you could import your Mass in to Revit and then create a Roof object from the Mass.
Those are my thoughts off hand.