The transcript itself cannot serve as a good summary for the corresponding paper, as it constitutes only one modality of the talk (which also consists of slides, for example), and hence cannot stand by itself and form a coherent written text.
to create an extractive paper summary based on the transcript, we model the alignment between spoken words and sentences in the paper, assuming the following generative process: During the talk, the speaker generates words for describing verbally sentences from the paper, one word at each time step.