I think I disagree on the flight having been canceled. I'd have assumed the distinguishing factor between a flight cancellation vs. rescheduling isn't the flight number (a flight number seems like an immaterial bookkeeping abstraction?), but rather, whether a replacement flight offers seats to all of the passengers of the first flight . It seems they did this, as they say in their text message. As far as semantics is…
Regardless of dancing around the semantics of cancellation, a severe problem was discovered in an aircraft forcing it to reland and delaying all passengers involved. If such an event is sufficiently costly for the airline then safety and maintenance measures will have to be increased.