Ridgelines in the sense that you have shown pictures of were certainly in use for a looooong time, but they do not set the sag. Like a tarp ridgeline, they hold the netting up and out of the way. Those ridgelines have nothing to do with the sag of the hammock.
Now if you want to define those ridgelines as "structural" for your own use, fine.
But you will speaking a different language from most everybody else on the forums. But then maybe that is the problem - some people want to "define" a "structural ridgeline" as something different from common usage. Then they want to argue over over it.