Definition of a lie: a false statement made with intent to deceive.
They make the point that, without looking into Trump's head, they can't know that Trump intends to deceive.
But what option does that leave? When Trump claims he won the popular vote, if that's not an intent to deceive, that leaves him being insane. (He cannot just be wrong about something so big.)
So, while NPR isn't incorrect in this, I think they're splitting hairs and doing a disservice in doing so. Call a lie a lie... or report Trump as being insane. One of the two.