I don't have the technical answer but I think the assumption is something like that. If you shoot a bird 300 mm away using a telephoto lens, and compare it against shoot the same bird cloer using 85 mm lens. The nearer to the subject, you are likely to get a clearly details of the subject.
Of course in some situations, you got no choice but to use a zoom lens and that is better than nothing. Hope I am correct.
Again, you are right..........to a point. It's hard to explain because so many things come into play. Now days, telephoto lenses are better than they used to be, so that helps. But, the number of megapixels your camera is capable of, can play a part too. If you use a good telephoto lens on a D800 camera (36mp), and a good 85mm lens on a 12mp camera, you'll get more detail from the telephoto lens on the D800. That's my understanding of it anyway...............
Sorry, but I don't mean to argue with you. Just trying to make sure I understand it correctly, and if so, try to explain some minor differences and the impact they make.
If someone else knows better, tell us so we'll know who's right, and which path to follow. Thanks.