There are tens of SE sites that deal with programming in some way, shape or form. SO is obviously the most general one of these, but there are many more specialized ones (CodeReview,TeX,Programmers,Drupal/Wordpress/Joomla/Server Fault/Ubuntu/...). If this site had been "Makers StackExchange" or "Digital Fabrication" then your question might have been appropriate (this site being a more general one), but "3D printing" should be seen as a more specialized one (but if you think there is a need for a specific Makers SE or a CNC SE, then you could consider going to Area51).
This site is called "3D printing" and CNC milling is definitely not 3D printing. I think it does very much make sense to limit 3D printing to "additive fabrication". This is not a particularly narrow scope, since there are many types of 3D printer, and not all of them use G-code. Powder printers (binder jetting) and DLP printers are driven with raster images, for instance.
If the scope were extended to cover your question (which is about 2D milling, not even 3D milling, while we're at it) then I wonder if that meant that questions like "How many flutes do I need on my 8mm ball-nose left-hand bit when cutting unobtainium with my feedrate at the speed of sound?" would also be in scope. When I hear "3D printing SE" I would definitely not expect that.
I would add that your question might have been closed on a hypothetical "CNC SE" as well for a variety of reasons: