Of course it's not a "how to get healed of stuff" book and simply saying to sick people "go garden a bit" would be terribly insulting.
That said, it is true that sometimes, what ail us is a lack of sunlight and watching things grow is good for everyone.
The lesson to take, imo, would be "Do what you can and what makes you happy". If you can go outside do so and breath the pure moor air. If you can't get a small potted bulb that you can care of and watch it grow.
Secondary lesson: don't spoil children having them believe they're little Rajahs.