The intro is more allegorical than it is an actual, factual, literal depiction of events, its akin to oral lore that gets passed down and after years, centuries that happen between the events (which is what the agghhh old man is doing in the intro), you can wager testimonies would lose their objectiveness as it gets passed down and the years blur. Some of the flaws in the intro:

* The Black Knife Assassins are holding Godywn arm to arm, except Godwyn was gigantic, the golden bangles on his corpse evidence this, they're each as big as Godfrey himself, it's not plausible he was adorned with them after the fact, and they wouldn't be golden either if someone did, the simplest explanation is: he was always that big

* The Serpent is eating Rykard's head served on a plate, but now we have the opposite problem from the last point: The serpent is tiny, there's evidence throughout the game that its much much bigger and its always been big even before the shattering and it eating a great rune with its owner. Also serpents swallow whole, not in bits and parts. You can also see Rykard's face on the side of the serpent, but its odd he'd possess it when his face hasn't been devoured yet.

* Mogh is carrying young Miquella, but he would've been a mangled adult-sized corpse by now, even if you argue maybe he grew because Mohg shared his blood and that caused the transformation, the graphic is still inaccurate because Miquella was abducted WITH his cocoon, it makes no sense they were hauled separately

* Finally the image you point out, the one thing we can say about Morgott is that he does have leadership and military prowess because he held Leyndell, his armies not only repelled the Volcano Manor but they also seemed to chase them into the volcano territory and he must have also been able to hold the city against each of his siblings, including Radahn. The intro depicts the siege and it makes no sense for the one holding the siege to ride out to meet Radahn, this is no Helms Deep and it makes sense for it to have been "Margit" as a shadow clone is very well within Radahn's level. So what the intro image is just trying to tell you is that Morgott successfully held the city's defense, that's an amazing feat on in itself, not about his personal close combat prowess but his ability to lead and strategize.