That's trivially also true for humans, you can't get out of your brain something that isn't in it.

If you meant "external training set", then it's false for both AI and humans, as demonstrated by e.g. AlphaZero getting superhuman performance despite zero examples of human games of Go or Chess.

the question is whether the cost of using AI to generate a solution, and then fixing it manually, will be lower, or higher, than building the solution using humans.

as we have seen before with wizards and other tools to generate solutions, the cost of fixing the solution is often times higher than it is to just build it from scratch manually.

also, as humans use AI more and more, there will be less and less material to use for training that is high quality, and AI will train on itself, leading to a rapid decline in quality of the output.