It depends. Being wrong isn't lying, and often people believe they are right no matter what. For instance, when somebody informs me "I'm really good at reading people!" they probably aren't, in my experience. But they think they are, so they aren't lying.
If somebody knows they are ignorant on a subject and decides to make a "guess" they know is a guess, but present it as truth, I think that's basically lying or at least misrepresentation.
So "David is a really bad worker and will get fired soon" isn't lying if the speaker believes it, but is probably rude to say and makes the speaker look pretty bad if David is later proven to be a great worker who gets promoted, not fired.
But, "I saw Melissa out with a guy. She's cheating on her husband!" when the speaker knows they don't know if the person Melissa's out with is a brother, gay best friend, straight best friend, coworker etc. and is just being gossippy/speculative but stating it like fact? That counts as a lie to me.