I see Executor class stah destroyahs. Therefore I agree with your analysis.
In all seriousness though, your conclusion is what I had assumed all along. One itsy bitsy biodome feeding an entire planet is ridiculous. Planets are BIG. Zoners feeding the Corsairs is real... but not with their gardens. There's a reason you see them in the houses, you know...
Oh, and you assumed the Outcasts and Corsairs were in step... they're not. The Corsairs have a lot more people than the Outcasts, the Outcasts just live about 2-3x longer (on average) due to A) cardamine effects and B) being background men rather than "YAHARR ME MATIES" pirates. Or at least that's how it was in vanilla, a lot of Outcasts seem to have thrown caution to the wind in disco. See Tridentes in Liberty.
EDIT: If Gran Canaria had the farmland available to feed the Corsairs, why haven't the Corsairs mounted a massive assault on it? Solving their food problem- permanently no less- would be worth losing ground to the Outcasts. I can see Gran Canaria helping, but I think that claiming it's well-farmed is not something the Zoners really want to say...