X happened, and then Y, so Z policy was effective is a common way for writers building a narrative to gloss over the fact that the two things linked may not actually have a causal relationship.
For example, X slew Y, becoming king is pretty clear: the killing of the old king allowed the new king to take his place. But consider “X brought peace to the realm by lowering taxes, negotiating with his barons, and concluding several alliances with his neighbors.”
It sounds straightforward. But can we be sure that the king’s policies were the direct cause of peace? Maybe the weather was good for several years, raising crop yields and giving everyone enough that they didn’t have to fight for resources. Maybe the king was lucky in getting a generation of barons who were more inclined to bend the knee than take control. Maybe the king’s neighbors were busy fighting civil wars, and too preoccupied with internal matters to seek outside enemies. Maybe all three things happened, and if any one of them had been missing, the kingdom would have been plunged into chaos.
Especially when reading condensed histories, we have to be aware of the perspective of the author, and what sort of point they might be making, even unconsciously, with the way they frame the story.