We already know that Island Sanctuary won't be providing crossbreeding of items currently in the game because they are a timed activity and YoshiP has specifically stated there will be no timers involved in Island Sanctuary.
Will there be things we can grow? Almost definitely. Are they going to be the things that create the current demand for houses with respect to gardening? Not a chance. It's the market competition that drives the current gardening demand. Another thing YoshiP has told us is that Island Sanctuary will not be competitive in any way (not that players won't try to find ways to make it competitive). That means whatever we can produce will not be things that can be sold on the marketboard since the marketboard is the most competitive content in the game.
Players are still going to demand houses for gardening because they're still going to want the items that come solely from gardening. Island Sanctuary isn't going to fix that.
People have their expectations set too high for what Island Sanctuary might be when YoshiP has told us repeatedly to lower them. It doesn't help that players continue to go around saying "you'll be able to do this with it" when "this" contradicts what we do know.
What do we know for certain about Island Sanctuary?
It is farming themed, inspired by games like Stardew Valley, Harvest Moon and Animal Crossing but much simpler in nature with fewer features.
It is primarily solo content.
It is intended to be relaxing, stress-free, non-competitive content that the player can enjoy when they want and ignore when they want without progress being impacted (hence why no timers).
We will be able to have our minions run around our Sanctuary.
We will be able to place buildings.
It is scheduled for release in 6.2.
There's probably one or two other things he's told us that I can't remember off the top of my head but that should be most of the information shared so far.
Where do you get gardening out of that?