With long cantilevers (i.e., supported at only one end) on both, to boot. Long cantilever, thin material = slop.

Even a small bench top lathe has hundreds of pounds of cast iron in the bed and ways, as you said.

That aside, a lot of the jobs they show in the video (pen-plotting, wood-burning, 3d printing, etc.) don't really demand a huge amount of rigidity. I wouldn't want to try any serious milling or turning with this machine, for sure, but for those jobs it looks like a totally reasonable design.

And I do like the fact that it's designed to be 3D printed itself (other than the extrusions and stepper motors, etc.), so well done on that.