Mostly the letter "B" for "ballistic".
You can turn while flying. You can't turn while falling. You can stick out a little control vane to try to change course a little bit, but at that sort of speed, in that sort of atmospheric density, and bearing in mind that during re-entry anything that is at all pointy will get burned off, I doubt you could do much at all.
That this was a "plane" is inconsequential, and not even interesting, in terms of missiles - that they had at least minimal control at that speed is either a breakthrough, or duplication of existing techniques that can be applied to big-ass missiles (there, swap the B with "Big") more easily and more cheaply than to something designed to be re-used. Which, given the difficulty in adding controllable movement at such speeds to something way more complex than a one-use missile, implies to me that the techniques have existed for a while in cheaper forms, and this is just the first time they've been able to do it and (nearly) recover the device.
That's significant, but it's not a game-changer for missiles or making things go boom.