Not Joe Stupid -- don't confuse ignorance with stupidity.
Your fundamental point is right, though, which is why it's highly irresponsible to sell these sorts of devices to ordinary people unless they're part of a larger system that imposes some sort of security.
"yet everyone here is claiming no such thing is possible"
They are? I think people here are saying that no such solution is currently available for naive users, which is true (and IoT manufacturers have zero interest in making one, because similar efforts in the past have sold badly). That's not the same as saying it's not possible.