Rule reminder
Cleft sentences manipulate information structure by dividing a sentence into TOPIC (given/known information) and FOCUS (new/emphasized information). In a cleft 'È Marco che ha vinto', the presupposition is 'someone won' (given) and the focus/assertion is 'it was Marco'. Pseudo-clefts work similarly: 'Quello che voglio è riposo' presupposes 'I want something' and focuses on 'riposo'. Understanding this helps learners use clefts appropriately in discourse.
In the dialogue: 'Chi ha vinto la gara?' — 'È stato Marco a vincere.' — Why is the cleft appropriate here?