Many vinyards do not convert all of their grapes to wine. Some don't make any wine at all. They grow the grapes and sell them to wineries. Some wineries grow no grapes, and buy all of their grapes.
Think of bread. A large bakery, like Sunbeam or Wonder may grow wheat, mill it, and bake bread. A smaller bakery may buy the wheat from farmers, mill it and bake bread. Smaller still would be a baker that buys theflour from the miller and bakes bread.
In our case, the kit maker is the miller who buys the grapes from the vinyards of the world, extracts the juice (mills the wheat), and sells it to us to make into wine (bake into bread). These are the same grapes that commercial wineries use to make their wines.