The fact that some understandings change, are clarified, or are in doubt doesn't imply there aren't provable lies. Statements can be said to be on a spectrum of provable truth to provable lie say from 2+2=4 to 2+2=17 and on a spectrum from neutral to harmful.

If I say ziptechnologies is Michael Jackson living in hiding with the mom of one of the neverland kids I'm provably lying. There is no legitimate doubt as to his death and no reason to believe that is your actual identity. If I say you are a drug dealer and invite the police to raid your home I have crossed over from neutral to harmful.

The fact that many statements can't be evaluated so simply doesn't mean there aren't obvious lines that many statements clearly cross articulable objective standards.