That depends on whether they are capable of creating a billion dollar startup.
If they aren’t, and I say they aren’t, then I am correct. If they are, and the AI’s output says they are, then AI’s output is correct.
If they aren’t, and I say they aren’t, then I am correct. If they are, and the AI’s output says they are, then AI’s output is correct.
Someone who really, really wants to make a billion dollar startup against all odds is going to ignore your advice anyway. In fact, they would ignore any AI's advice on the topic as well. But that kind of arrogance is precisely what's required to be able to pull it off. Someone who quits the moment an AI tells them "don't do it" was not cut out to accomplish such a goal to begin with.
And maybe in the end the startup will only be worth a couple million dollars, but the hubris to believe they could go even further would be what got them that far at all. So "can build a billion dollar startup" ended up being false, but something else was gained in the end.
EDIT: To be clear, I somehow doubt an LLM would be able to provide the level of support needed in most scenarios. However, you and others around the potential founder might make the difference. Since your assessment of the person likely influences the level of support you provide to them, your assessment can affect the chances of whether or not they successfully build a billion dollar startup.