If it is a credible bluff, does it work?

No because it is never a credible bluff. You would not be having the conversation if it was.

In fact having sold stuff If a lead says this, it is a huge red flag for me that I probably don't want to do business with them because they are probably a "vampire customer"

LLMs can write surprisingly decent code a few hundred lines at a time but they absolutely can't write coherent hundred thousand line or bigger programs.