I am aware that 단 말이야 is often used to confirm information that the speaker said. All of the practice examples have what the speaker said roughly verbatim.
나: 어제 10시간이나 잤어요.
가: 네? 어제 10시시간이나 잤단 말이에요?
In the following example, from a k-drama:
나: 자살이 아니라 타살이라면 범인은 저 문으로 들어왔거나
가: CCTV가 없단 말이지?
I am trying to understand if 가 is drawing his own conclusion or is doing a sort "So what you're saying is...". Since this subtitles translating could mean either
-단 말이지? (and its original form -다는 말이지?) is a way of confirming what the other person said by asking again rephrasing the remark in one's own words. Sometimes it can be asking back because you didn't get it the first time but I think we typically say it to get a better understanding. In simple cases, this rephrasing may be considered a conclusion, but I don't think you can say the phrase is about making a conclusion.
And when asking back because you didn't hear it or because you can't believe it, -다는 말이야/말이에요? (as in your first example) or -다고(요)? is the more common form.
Your second example does sound like drawing one's own conclusion, but to me it sounds like a special case. It sounds like 가 is thinking deep surmising clues from what 나 said (as detectives do) but it is not a typical case. If you just realized something from what the other person said, we are more likely to say -단 말이구나 or -단 말이네. These endings express just such a sense of new realization or discovery.
