What do you mean? Lathes have been used to do subtractive manufacturing with metal for thousands of years. Modern Lathes are CNC types and can build a lot of stuff.
You do not want your frame to flex while a spindle is cutting aluminium at 20kRPM. See Marco Reps and Kris Temmerman on youtube, they built formidable high precision CNC mills out of steel and concrete. For machines that rely on frames made out of extruded aluminium, flexing has always been an issue, even with much sturdier designs such as the Avid CNC machines.
Also, FYI, in a lathe the workpiece spins, and in a mill the tool spins.
One main reason professional 5 axis CNC machines are so expensive is because they are very rigid yet can move quickly and precisely.
[0]http://millingaccessories.biz/2015/04/07/maxnc-10-cnc-millin... [1]https://lh3.googleusercontent.com/-TkXnpUUXqLg/T4Qj3qVU9JI/A...
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.