It is worth noting that the question referenced in the OP deals with a vacuum forming machine, which isn't (similar to) a 3D printer by any stretch of the imagination. It heats up a sheet of plastic so it melts, which is then formed (using a vacuum applied from below) over a model. This just allows you to make copies of an existing model.
If we want to allow these types of questions, we should get the name of this site changed (to something like "Makers SE"/"Rapid Prototyping"). I don't know if that's even an option, but as it stands, "3D printing SE" should be about 3D printing; CNC mills, vacuum formers, etc... are clearly not 3D printers and thus off-topic.
I don't find "there's no other place for this" to be a convincing argument; the scope of a site should not depend on what other sites do or do not exist.
That said, I do feel the just 3D printing scope is a bit limited and we could attract a larger audience by including other types of rapid prototyping/manufacturing. The name would be a huge misnomer if the scope included that, but I guess the only way to get a name change (if that is possible/appropriate at all) is to begin by changing the scope.
As it stands, the question referenced in the post is definitely not on-topic (since the machine involved is very un-printer-like), but questions about CNC machines (and similar machines) might be. They mostly run different softwares (GRBL/Mach3) but you could conceivably run a CNC machine on Marlin.