Source: p 34, Understanding Semantics (2 ed, 2013) by Sebastian Löbner
Interjections and exclamations can be used as complete utterances. Other expressives such as hopefully, (un)fortunately or thank God can be inserted into a sentence in order to add a personal attitude to the situation expressed. These additions do not contribute to the proposition, as the following examples illustrate:
(11) a. Fortunately, Bob will arrive tonight. — Really?
b. I'm glad Bob will arrive tonight. — Really?
The questioning reply 'Really?' is always directed at a proposition asserted before. [1.] In (11a), the remark can only be related to the proposition that Bob will arrive that night ('Will Bob really arrive tonight'), not to the speaker's attitude expressed by fortunately.
How is 1 true? I ask not about what is prescriptively idiomatic. For example, what if in uttering 'Really?', a speaker means >Really? How is this fortunate?< but without uttering the second question?