Well, you can run servo’s on Marlin, and verify positions and movements. Getting WAY over my head here, and I’m not sure of anyone actually doing this.
There’s a number of other conversations about “Marlin servo”, and the codes been in the firmware for awhile now.
As far as just “running the g-code like a punch card”, as far as I know, even the “mid-range boys” use CAD to design, CAM to generate the code for the machine, and then they feed that to the C/NC to make shit happen. I can stick code in place to do tool changes, even with an ATC, AFAIK. But the computer that calculates all the movement is not the same computer that’s controlling the machine. Again. Could be off base on that. Just having a random/off-topic discussion. 
Then to confuse things more, I’ve seen a number of LinuxCNC based machines, running simple NEMA23 motors (no servos or encoders). Does that make them an NC or a CNC? For that matter, would anyone “in the industry” even understand when you tell them you have a Shapeoko NC making signs for you in your garage? Or would they correct you to say you’ve got a Shapeoko CNC?
C