The Twin Otter is what it is, a 'heavy' under 12,500 pound general aviation airplane.
I operated the first one on the west coast of the USA in 1966, a -100 series model.
A nice airplane, no doubt about that, but still....not a transport category machine.
SFAR23/commuter category, notwithstanding....just a perfectly safe larger 'small' airplane.
I think the FAA went way overboard on the commuter category nonsense with the DHC-6.