3D Printers usually have support columns that are easily taken down once the print is complete. Maybe something similar?
https://m.all3dp.com/2/cura-support-settings-optimize-your-s...
Looking at the photos of the houses, I wonder if they pause the printer at the point just before where it needs to draw the first layer above the top of the window frame. At that point a couple of workers manually lift the prefab window unit into place. Then the printer can resume and print that next layer on top of the top of the window frame.
That said, wooden and, more recently, foam forms have been used since forever. (At least a hundred years?) Breaking the forms away from cured concrete doesn't seem to be problematic.
EDIT: Also, in another video there's a brief shot where the machine is printing the internal zig-zag pattern over a steel mesh manually placed on the wall, presumably for reinforcement. I fast-forwarded through 4 or 5 videos and it's almost suspicious the way they avoid capturing these details. I guess such details don't fit the narrative they're selling.
There's got to be something in there that supports horizontal forces.
Personally I think they should emphasize the manual labor requirements. One of the strongest criticisms of 3D-printed and prefab housing technology is that it reduces demand for low-skilled labor. For the past 80 years home construction has been a major pillar of the American labor economy; likewise for China and many other countries. Free or reduced cost housing is great, but to the extent it comes at the expense of a sustainable labor economy, then it's problematic.