I was the one who posted that statement.
To answer the last question, yes, you should run a strap from the cab to the frame. This is to assure you have ground for your tail lights. They ground to the bed which is attached to the frame. When everything is new this is sufficient. However, once the bed bolts start getting rusty it affects the grounding. That is why I also like to run a strap from frame to bed.
As to why the factory manual doesn't show a ground to frame I can't answer. Perhaps because when it is new there is an effective ground through the cab mount bolts. But, like I said about the bed mounts, that connection will deteriorate. I like to use redundant systems to assure good grounds. I don't like troubleshooting electrical systems.
