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The way I see it, there are 3 possibilities - none one of which results in any significant changes to the setting we play in.

A.) Scenario A: Multiverse. An Ultramarines cruiser (or any other chapter, because deep at heart they all wish they were Ultramarines anyway) goes back in time, right before the Horus Heresy. They warn the Emprah Emperor and stop the Horus Heresy. In doing so, they create a new reality, in which the Horus Heresy doesn't happen. Unfortunately, we play in this reality, not in the alternate reality. So lets presume that has happened, even, in one of the infinite universes that exist. We play in this one, Horus Heresy occurs, and only Uatu knows of the possibilities.

B.) Scenario B: Tzeentch. Lets say there is no multiverse, and only one reality. The Imperium's technology is clearly not going to be fractionally as masterful at manipulating the warp as the Changer of Ways, a warp god. Hence, this possibility is explictly prevented by Tzeentch, as he likes our current screwed up reality. Horus Heresy occurs.

C.) Scenario C: Plot. Unfortunately, the one truth in the warp is that every vessel moves at the speed of plot. As GWS doesn't wish it to happen, it will not happen. Horus Heresy happens.