For one, saying "I'm not ready" isn't always a lie. Both "I'm not interested in you" and "I'm not looking to date anyone right now" can be true.
You're right, I should have made that more clear.
It was also especially hard to say directly "I'm not interested in dating you" when he never actually explicitly would say it. Well around 4 months later I started going out seriously with me now-fiance and he just about lost his mind. I hadn't been looking to get into a relationship but it just ended up being right with him. The NiceGuy started bad-mouthing me to everyone he knew, all our friends, and saying that I "led him on" blah blah blah.
How does this not prove my point? You lied to him about your feelings because you felt it was "hard to say" and now you're super surprised that he's salty when you demonstrate that it really was him, not you? I'm not defending his actions, he sounds like a shithead and you absolutely do not deserve anything that he did to you in retaliation. However, I think had you been honest and worded it diplomatically, he would not have acted that way. It would have been painful in the moment but over time it would have been easier to reconcile than you demonstrably lying to him.
Also, there are always varying levels of honesty, no matter what. You may think that partner was being 100% honest but who knows, maybe there really were things she was holding back to spare your feelings. When I broke up with my ex I told him "I don't think it's working out," not "You're a narcisisstic, self-centered asshole with a fragile ego and a super weird relationship with your sister, and who will die alone" because I don't want to be crueler than I have to- even though it's true.
I'm not telling the whole story. You're right- you never can be absolutely certain whether or not someone is lying about their emotions/feelings about you. There's a lot of details I'm omitting and it was kind of an unconventional situation, but essentially I've analyzed the situation and can really think of zero reasons why she would lie. Who knows though, in any case I'd rather by wrong and clueless than right and even more hurt.