"Unreliable narrator - yes, because any author who writes a novel about a historical event is unreliable"
Even witnesses are unreliable, yes.
That is not what I meant, though. I used the literary term Unreliable Narrator to mean a narrator whether in literature, film, or theatre, whose credibility has been seriously compromised - as in, a narrator who is mentally ill, or is deliberately distorting events, or has a cognitive disability. He either doesn't understand what he sees OR he is likely to lie to us about it, so we can't believe what he says.
Laurent Bidet is an unreliable narrator simply because he makes lots of stuff up AND tells us he does. Therefore, what he doesn't say he made up is hard to believe.
He also regularly says nonsense like "I’ve said that I don’t want to write a historical handbook. This story is personal. That’s why my visions sometimes get mixed up with the known facts. It’s just how it is." HIs visions??? 
"’I'm not sure yet if I’m going to ‘visualize’ (that is, invent!) this meeting or not. If I do, it will be the clinching proof that fiction does not respect anything." - No, Laurent. It's not that "fiction" doesn't respect anything. It is that YOU don't respect the actual historical event and are confabulating.
And he agrees with me:
"I’M FIGHTING A losing battle. I can’t tell this story the way it should be told. This whole hotchpotch of characters, events, dates, and the infinite branching of cause and effect – and these people, these real people who actually existed."
"Our worth should be measured by our aspirations more than our works.’ That means I’m allowed to make a mess of my book" - Err no, Laurent. You are not 
To each their own and all that but I am
that you actually liked this book and honestly think we should stick to recommending non-fiction to each other in the future 