The same way you know that the things you learn from a person isn't made up gibberish: You see how well it explains a scenario, how well it lines up with your knowledge and experience, and you sample parts to verify.

how do you know they didn't learn from the garbage generator too?

You are literally describing the fundamental problem of truth in philosophy and acting as if it's different because a computer is involved at one step in the chain.

Because it works.