What's the point with that quite lackluster story in Endwalker?

[Absimiliard]

IV Golbez was, though brainwashed, still the one in the proverbial pilot seat. All the manipulation and subterfuge was his own idea. What Zemus did was prey on Golbez's insecurities and eat away at feelings for his family, which in turn eroded his sense of morality. After that, it was simply a matter of giving him a directive to follow. Everything that came after was Golbez's own skills at work, albeit toward an end he would never have sought if not under the thumb of another.

The crappy 3D version of the game has a couple flashbacks in it that give some details on how things went down. Then you've got the After Years, which contains even more information on Golbez.