I can't imagine it's ever fixed, at least not any game that has a speed attribute on characters. The difference between a real-time combat system and a turn-based combat system is that in the real-time combat system, the system will act without waiting for the player.
That being said, with what we see in FFXIII, it seems entirely real-time. Not only is the order of actions determined in real-time, but also the results are affected. You can interrupt attacks and your attacks can be interrupted. The positioning of the models on screen effects the timing, which effects the chaining, etc. It uses an ATB system, but the pacing seems to be very similar to FFXII's pacing. I would have to imagine that the party members would be controlled by AI based on menu settings or something, it already seems hectic enough to control one character at a time at that pace. This does definitely seem more like FFXII's battle system than X or X-2's to me, I think the fact that battles are not seamless is what throws people off here. Familiarity with WKC's battle system probably makes it easier to bridge that gap. This game looks like it took WKC's system, which had great ideas but relatively mediocre execution, and perfected it.