The problem is that a native sign-language user would have no idea that two English words rhymed

I do agree it’s a fraud, but I’d also say “native sign language speaker” is a bit of a misnomer. Comparing it to a human we’d be talking about someone with perfect hearing but born mute. IE if they were taught by someone who spoke out loud as they signed, they would understand ryming in ways that someone who was deaf obviously wouldn’t.