"If it fits, it works" is pretty easy to explain. An adapter cable is also easy to explain: it fits, so it will work. With USB C, there are scenarios where you can have an adapter cable and it'll still not work.
If I recall correctly, DVI had three main variants: -I, -A, and -D. A DVI-to-VGA adapter would have a DVI-A plug. The video card would have either DVI-I or DVI-D. If the card had DVI-D, it only offered digital and the DVI-A plug would not physically fit. If the card had DVI-I, it would offer both analog and digital and both DVI-A and DVI-D would fit. So, if it fits it works!
And yeah, the HDMI thing is of course true, I'll give you that. :-)
The pin helped sometimes but there were a few arrangements that fit but did not work. Female DVI to Female VGA adapters existed for going from VGA to DVI-A but this meant you could fit a pinless DVI-D cable and a VGA cable together this way.
