A cyclone in Australia is defined as, "having a maximum mean wind speed of 34 knots or greater, extending more than half-way around near the centre, and (those speeds) persisting for at least six hours."
BOM - About cyclones
On that basis, Olwyn was still a cyclone until well inland from Carnarvon. It petered out into a rain-bearing depression NE of Geraldton.
The reference to "ex-tropical Cyclone Olwyn" was probably confusion by the journo over the cyclones definition at the current time of writing the article, and the time when Olwyn actually hit Carnarvon.
Carnarvon registered peak wind gusts of 146 kmh (79kts) at around 13:30 on the 13th March.
Carnarvon - last 72hrs of weather observations