That's simply untrue. If someone says something that is untrue, but they believe it's true, they're not lying.
Your reasoning reminds me of the late Ross Perot. Perot was fond of saying, in regard to complex problems, "It's really very simple." In fact, what he was talking about was invariably not "very simple" at all.
This has been one of the worse features of the coarsening of public discourse: the rampant use of the term "lie" as a substitute for "untrue" or "falsehood."
Of course people passionately hold different beliefs and conclusions. But the new discourse is that, when someone says something you believe to be untrue, now you say "
that's a lie!" rather than "
that is false."
By using the term "lie" you take away the benefit of granting the other person may...just like yourself...actually believe what he says.
And calling something a "lie" also lends the user of that term the juicy sense of moralizing at the same time - something you don't get by just pointing out something is false.
I would hope this trend of automatic moralizing and giving people the least benefit of the doubt possible subsides at some point, but the increasingly antagonistic divisions in society don't bode well.
(Often enough some people will want to defend the use of "lie" by saying "look, he/she HAS to know what they've said is untrue!" But that often rests on a naive take on human psychology - we really, really can get ourselves down cognitive rabbit holes and the "sense" you think you are making isn't what makes "sense" to the other person, unfortunately).