It is good that you found that original question. It was added to the What's on topic meta question, but that area of the topic list doesn't seem to have been particularly well hammered out. If you look at the What's on topic help page, it does state:
Blender - whilst there may be some overlap with SE.Blender, if the question is related to 3D printing then it is on-topic
Maybe the whole line needs re-wording to be less Blender centric..?
Regarding the question that caused this issue to be raised again, How to combine difference with concatinate object, I must admit that initially I agreed with Mick's comment that it was probably on-topic. However, following your comment, I wholeheartedly agree that any CAD question really should have some 3D printing element attached, in order for it to fit on this site (as the On-topic help page implies).
So, in short, you are indeed correct, and backed up by the what's on-topic help page.
In addition, the on-topic help page needs a slight clarification, such as:
If a CAD question is related to 3D printing then it is on-topic. However, general CAD questions, if there is not a 3D printing angle, are not on-topic. Likewise, with Blender - whilst there may be some overlap with SE.Blender, if the question is related to to 3D printing then it is on-topic.