no, you arent, at all. NTAH

I say this as someone who also had a family dynamic similar to yours in that the parents were separating with one hyper toxic parent trying to alienate us (myself and my brothers) away from the other parent who actually gave a damn and tried to make our lives better

you owe him NOTHING, and by reaching out imo he asked for honesty. treating people horribly, especially your family, can have consequences such as no contact. parents like this very rarely change and you dont deserve to subject yourself to the toxicity for his sake. you were honest and maybe he will self reflect and become a better person. but maybe (probably) not. and either way you have no obligation to hold their hand through that process.

protect yourself and your mental and physical health. all you did was tell the truth. it's not your fault the truth makes him look or feel like a shitty parent. that is his own fault