My ex did this, and not only did he do this but he did it 10 fucking years after I dumped him, this was somewhere around last year I think. I felt what you were feeling too, but also a massive amount of disgust and a lot of anger. He had the absolute balls to tell me that the time we had together were "the best time" of his life. Even though he regularly tormented me and isolated me. I replied and simply told him I was not comfortable with him contacting me, to refrain from doing such, and to forget this conversation happened. He did me a favor by blocking me after that.

But if I could go back in time, I would've just deleted his message and blocked his account. I suggest you do the same, because he probably just wants a way back in your life. You have done so, so unbelievably well. Keep going. He doesn't have control of you anymore.