Earlier quoted context omitted.
The reason I can say my smarthome is relatively secure is because I don't allow any of the devices to access the internet. IP or not seems irrelevant.
The high level of sophistication of IP networking and the fact that one small mistake could mean your devices access the Internet is why a network that doesn't talk IP is almost inherently safer than one that does.
It may seem trivial for those of us on HN to block IP devices from the internet, but the average user will not know how to do that effectively. They are also the ones who’s devices probably remain on a botnet undetected by their owners.
That said, most hubs (required for these types of devices) connect to the internet by default anyway. It does reduce risk a bit as vulnerability in a node isn’t exploitable remotely.