I believe you misread the OP. He's clearly describing the behavior of ACC, one which I have encountered many times . He did not let go of the brake, he did not disregard the notification. He did not engage AEB. He did not "rear-end" anyone. He's just wondering why the system doesn't work the way it seems like it should, and then describing what would happen if you don't take over braking.
Saying it "works differently" is not really an explanation, it's just an excuse for a system that's not as useful as some other ones, including ones in other Kia models. Blaming driver oversight is unwarranted for someone simply asking a question.
It's conceivable that I misread the post, since it is worded a bit oddly, and that he actually did disregard all the messages and AEB activated, but that's reading a lot into it. Maybe OP will "chime" in and clarify.