True, I actually thought columbo had been honest when saying that lytton didn't really kill janie's father, but columbophile (who owns a blog with many columbo episodes' reviews) also thinks he lied when saying that, a thing I hadn't expected.

I think getting Columbo to say "I lied about her killing your father" was basically a "trade" for her confession. As she rotted away in jail, She didn't want Janie thinking that she was also responsible for her father's death. So these were the terms under which she would offer her confession to Columbo.

Of course he has no proof and it's all speculation, but he seems pretty sure that she is indeed a triple-murderer.