Agreed! But then how can the case mentioned by AtoMaki come up? When you have this level of evidence, there’s no way that you are wrong about it. So you won’t be in a situation where you want to trigger the technique and find out it’s not working...
Only possibility I see is if the Kuni had been willfully deceived by way of an illusion or similar means. Some Air spell made it seem like that merchant was performing Maho; try to activate the technique... and it fails. That fact only would let the Kuni that he has been fooled, and it’s an interesting effect of its own.