Do pressure-cooked vegetables really have nothing else to give?
After extended pressure cooking (or any liquid cooking, for that matter) the vegetables' cell walls (composed primarily of flavorless starch) have broken down to the point where they're barely holding together, and are basically mush.  Most of the flavor and nutrients contained within those cells has escaped into the liquid at that point.  There might be a bit left in the solids to extract, but very little practical way to do so. 

It's probably more accurate to say that the vegetables have given up enough that they're unpalatable. You could eat them, but they wouldn't be anywhere as pleasant as fresh vegetables, and would offer little nutrition separate from the broth itself.

I suppose you could puree them (though you'd be turning flavorless, starchy mush into smoother flavorless, starchy mush) but they have little culinary value left. At most they would be filling, but probably unpleasant to eat.

On the other hand, if you look outside of culinary applications, they make great compost.