Agaricus bisporus, DO 228 is NOT a transport category (FAR/JAR part 25) aircraft. It is certified by light twin rules, part 23. Some of them weigh more than 12500 lbs, but that only changes the rules to SFAR23. You may argue that in some respect it is "large", when over 12500lbs, but it is definitely not transport category, like DHC 8, which the passengers want back on that route.
So if I may rephrase: Has it not been established that transport category aircraft are safer than lighter twins?