Extraordinary Beef Gravy?
It's probably less to do with the amount of juices that came out of the rib roast and more to do with the extended caramelisation of those juices produced by overcooking. Those burnt, caramelised bits left in the bottom of the pan are full of flavour and it's probably that, that added so much more flavour to your gravy than you're used to.

Just replicate it the next time you make a rib roast. Once the roast is done, take it out, cut a slice or two of the meat off, chop it up into small pieces, put it back in the pan and continue cooking until the pieces are burnt and caramelised. Make your gravy as before.