I'm not sure that the word is quite right in this context but it basically refers to the fact that the event is deeply embedded in the national consciousness and that it appears to provide lessons that may not be correct ones. It's a variant of meaning 1. The sense in which that applies is probably best given by these paragraphs:

What were the lessons of Verdun, if any? In 1940, Hitler’s smart new generals, such as Heinz Guderian, tackled the problem of static warfare with a blitzkrieg technique of fast-moving armoured attack. They went round the side, and back, of Maginot’s costly fortresses, making them capitulate without firing a shot. Verdun fell in a matter of hours, France after a six-week campaign. But Hitler missed the main point, militarily: never make a fetish of any fortress, any stronghold, so that you become committed to hold it however unacceptable the costs.

The capture of Verdun in 1916 would have been unlikely to have caused France to lose the war. Yet in the Second World War, Hitler’s orders to his generals not to surrender an inch of ground, led to repeated disasters – beginning with the sacrifice of Friedrich Paulus’s entire 6th Army at Stalingrad, which eventually brought about the destruction of the Third Reich.

And yet, in the heart of the French army, the myths and the slogans of Verdun, chimeric as they were, persisted well into the 1950s. Brave young paras died at Dien Bien Phu in the First Indochina War with Verdun on their lips.

The "Legend of Verdun" works better than the "Legendary Battle of Verdun"...because the point of the article is how keeping Verdun is a point of pride for the French...and yet not only did it involve a terrible loss of life, it wasn't much worth fighting for. But the "legend" part is how the pride over keeping Verdun -- i.e. the idea that holding a fortress is important -- had echoes in later military blunders of pride, such as Hilter at Stalingrad and the French, again, at Dien Bien Phu.

I agree with all the answers that have been posted so far, but I think it is worth pointing out that, in idiomatic usage, an event or person may be described as legendary even when the facts are well-known.

The way it works is this: many historically-important legends have a basis in important events in a people's history, when bold and resourceful action saved the day, but the story tends to exaggerate the glory. A recent event may be described as legendary if it has those properties to a degree that is normally only seen in legends, and the same goes for the primary actors in those actions. An example that comes to mind is Shackleton and his rescue of his crew, a legend in the history of exploration.

I mention this because of the sentence 'Yet over the interwar years it justly came to be seen as France’s “finest hour.”' (my emphasis.) I think that here, the author is saying that the battle reached legendary status within the French national psyche, without implying that this is a misreading of history.

To be clear, I agree that if Maginot's enthusiasm for forts was a result of Verdun, the author is saying that was a misreading of history. As for Hitler's "no retreat" doctrine, I think the point is that he was, for his own reasons, making the same mistake as the generals at Verdun: I doubt that his attitude owed much, if anything, specifically to that battle (he was at the Somme at the time.)

In this case, I'd say it's a combination of definition 1 and 3.

The "story" of the Battle of Verdun is definitely quite famous and is part of the larger narrative around WWI. While it's obviously not a person, it has some of the legendary aspects that fit into definition 3.

In general, the popular definition of "legend" includes practically any aspect of popular history.

I was confused by that at first as well. A better title would have been, "The legendary battle of Verdun".