Maybe I don't get exactly what this means, but isn't it true that if see a message that someone's signature changed and the other person sees this also and we b

Maybe I don't get exactly what this means, but isn't it true that if see a message that someone's signature changed and the other person sees this also and we both choose to ignore this message the man in the middle can read all our new conversations? Ofcourse if then one of us checks the signature later and sees it is not correct this would be very harmfull for whatsapp. But this indeed doesn't sound like a backdoor. It's just the way it works. Which seems good enough.