Why is it that you can't understand that lying needs to have the knowledge that what you are saying is wrong?
He's absolutely right, Floo. Lying is deliberate deceit, not just being mistaken. Lying is to tell a conscious falsehood - to say something that you
know to be untrue by which you're trying to mislead your audience.
Being genuinely mistaken (and being quite sincere in your belief) and lying aren't the same thing.
In other words lying isn't the word you're after for what you're trying to put across.