Is A novel about a historical event a work of nonfiction?

Absolutely not.
Text books are non-fiction and give the facts as they are known at the time of writing. They speculate but they back up speculation with fact and reasonable assumption.

Historical fiction deals with the same events but dramatises them, and to do that you have to draw on the writer's imagination. Yes we have some historical character's own words to draw on, or at least, what has passed down to us as their own words. We have reports from chroniclers or the TV, but there are going to be conversations and events that happen behind closed doors that have to be imagined. And that is all fictional in a novel.

For instance, I know from my studies that Prince Llywelyn the Last died on December 11th, 1282 at around three in the afternoon. However, although I can put that in a work of non- fiction because it is a recorded fact from more than one source, if I want to expand on that, say, 'it was cold and growing dark, Llywelyn rode across the hills towards Builth when he met with a group of English soldiers. He drew his sword and with his own soldiers at his back he engaged with the enemy, striking at the first knight to approach him' then I would be moving into the realms of fiction. I don't know what the weather was like, but I can guess, and I don't know that he drew a sword, but it makes sense that he did. But it is not known fact. That level of detail is not known, but it sounds authentic and is based on my own personal knowledge of the event in question. But it is still fiction, until a source comes to light that states it was cold and getting dark when he met with the English, is will remain fiction.

And this also raises the point that a well written historical novel can be as informative and possibly more so, than the best text book because it presents the events and characters in a form that is far more real and immediate. If you can relate to the people involved and someone pieces together all the disparate facts from the text books then the historic world comes together into a 3D picture and not the 2D that non-fiction has a habit of doing. The more ancient the history, potentially the more valuable the well written and researched novel becomes.