Full disclosure here, I will never be able to afford a 1DX, or 1DXII. That said, what is the point of such comparisons? Does anyone regularly shoot at -5 EC? I'm guessing that most folks who've been around digital photography for any length of time realize that proper exposure is key at any ISO, and more so at higher ISO.
Throughout POTN, there are shots from 6 year old crop cameras at 12,800 that show very little noise. I've made shots from a 6D at 102400 (controlled settings) that showed very little noise. I find my 6D files relatively easy to work with a 25,600. A 5-stop increase would be ISO 128,000. Is that ever necessary? A 5 stop underexposure would be at shot at ISO 800. Who would do that on a camera capable of decent ISO 12800 JPG files?
Is this a useful comparison, or is it just semantics?
There's already a number of other good replies, but yes; essentially a high DR capable system means that you have the opportunity to hold some detail in bright highlights (i.e. not clip them) whilst still retaining usable detail in shadow areas. A fairly easy-to-test example of this would be an architectural shot, where you're taking an image inside a building (especially something like a castle or old stately home) and there are views out of the window on a bright day). The DR in that scene will likely be large, so to hold some detail in the view out of the window means you'll get a very dark exposure (out of the camera) and will then need to lift the shadows by several stops to get something that looks "right".
So, "proper" exposure really depends on the shooter's intention, mixed with the capability of the gear. On the 5D3 the proper settings would be to accept some clipping in highlights in order to obtain some usable shadow details (or bracket). On an A7Rii you'd get away with holding much more highlight detail.
Note that a 5-stop push on ISO 25,600 would be ISO 819,200. I.e. lots! I have pushed shadows on an 80D ISO 6400 shot by probably a couple of stops - pleasingly they were (just) acceptable for the intended use. Push a 5D3, 60D or 7D (mark I) file by a few stops and things tend to go bad fairly quickly.