Correct, RD-S holds a long lease on Tresco and owns the hotel on the adjacent island of Bryher.
There were up to 6 choppers a day a few years ago, although the Gardens on Tresco now get many visitors from visiting cruise ships.
Ultimately the chopper service is about operating in poor weather, which is why PZE is a better options than Lands End. The ISSCo initiative is a rather childish spoiler in my view and won’t last long, if indeed it really starts.