It's a Canon update.
It's VERY hard to get accurate AF beyond f/5.6 on any camera (ok, I exaggerate... maybe f/6.3). If you run tests on a body, you'll likely find that the camera focus faster at lower focal ratios than it does at high focal ratios. In fact there maybe hidden/extended AF points that only activate at lower focal ratios.
When you're doing AF at f/8, you're really on the edge. The camera may be doing it, but it's probably not digging it. Nikon released the D800 with AF up to f/8. Canon released the 1DX and 5DIII but stated the AF is only spec'd for f/5.6. SOOOOO... after all the screaming subsided, Canon worked on an update that will let the computer do AF at f/8.
In reality this is a bit of a physics problem and no so much a "those cheap bastards were trying to screw us" problem. Like I said... EVERY camera will struggle at f/8 whether the body claims to support it or not. If you really need lightning fast focus on your super long telephoto lens, the solution is easy... just spring for the ludicrously expensive f/2.8 glass and you'll have no problem. Sigma makes a 200-500mm f/2.8 zoom and it only costs $25,000. Sure, you say... but I'll need two extra friends in good physical shape just to help me carry it and I can't use it for "hand held" shots and I have to shop for a tripod at the army surplus store by looking for a turret previously used for a howitzer. Hey, we all have to suffer for our art.
Frankly, the guys who can really afford a 1DX can also afford the glass to go with it. But if you're buying that "budget" f/4 telephoto and you want to use the 2x converter (because buying the focal length you needed in the first place was more expensive than the total value of the first five cars you owned combined) then you'd find you were at f/8 and had no AF in normal mode (you could switch to live-view mode -- totally different focus system.)