Would someone still be considered a liar if they told information that they thought they knew was correct, even though it wasn't?
No. There's times that we feel certain about something we know. And when the info is invalid, it could be because of a wrong source & wrong teachings... Unless you know otherwise, you didn't lie... You expressed what you thought was right & valid.
But now you've been informed of your mistake.
No, they were misinformed, or mistaken. Been there, done that. Apologized and gave the correct info later.
I would think they would simply be "misinformed". A lie is something that you knowingly tell or purposely omit information from. They shouldn't be held accountable if they recanted everything that was--to their knowledge--the truth.