To avoid compensation EU Regulation 261 2004 requires that "all reasonable measures had been taken by the air carrier concerned to avoid the delays or cancellations..." A reasonable measure could have been to arrange for backup equipment or booking an alternative flight, even on a different airline. Delta obviously didn't do either. A similar situation occurred for me several years ago. Delta's flight from New York had a technical problem and was cancelled. They tried to use this as a valid reason for cancelling their returning flight from Milan to NYC, but eventually paid us €1200 when I convinced them that a) technical problems are not considered extraordinary events, and b) the technical problem they referred to didn't even exist on the Milan-JFK flight.