Earlier quoted context omitted.
No, the effect on supply is the same. well, no, if you're growing for personal use you can make a notable effect on your own supply/food costs. You don't have to solve the global food shortage to benefit from a personal garden and since the global food shortage will drive up prices, the financial benefit is even greater (as long as price increases in things like fertilizer don't eat up your cost savings).
You seem to be thinking about this from a personal finance angle instead of an economy-wide production angle. It doesn't matter if a piece of corn is made in your garden or on a farm. The net effect on the corn supply is identical. It takes orders of magnitude more input to grow a piece of corn in a garden than on a farm. That had better be offset by the personal enjoyment of the gardener.
Yes, I tried to be clear:
Depends whether you're gardening for a global food shortage, or to supplement your own use, which I suspect is why most people have home gardens.
No one's backyard garden in the USA is going to help feed someone in Africa, but even if the global food shortage doesn't mean food shortages in the USA, it's going to drive up prices, and a backyard garden can help offset that household expense.