[Source] [Slide 20:] An area has been shaded twice. So, we haven’t diagrammed the conclusion. The argument form is invalid.
[Slide 27 of 27:] No syllogism whose diagram places an X on the line or results in double-shading is valid.
I already understand, and so ask not about, the particular example in Slide 20. Instead, what are the big picture and intuition behind the title of this question?
Please do not answer with formal proofs or arguments.