It’s not really in there; as Andrew wrote there’s an open issue, and the same problem is discussed here on the forum: How to access the elements of a bibliography?
Why is it not really in there? Because Typst first has to create the content from the bibliography, and that is not necessarily possible immediately. With IEEE for example (i.e. “[1]”), Typst has to collect all the citations to know what the correct numbering is and which reference gets a [1] and which gets [2] etc.
Effectively, cite() is therefore similar to context in that you can not inspect its content (see Why is the value I receive from context always content?); you can however inspect the input given to produce that content, which is what repr() shows you.