-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.