> Suddenly, your smart lightbulb is accessible by everyone. Not a great idea.

The answer here is kinda that Wi-Fi isn't an appropriate networking protocol for lightbulbs (or most other devices that aren't high-bandwidth) in the first place.

Smart devices that aren't high bandwidth (i.e. basically anything other than cameras) and that don't need to be internet accessible outside of a smart home controller should be using one of Z-Wave/Zigbee/Thread/LoRaWAN depending on requirements, but basically never Wi-Fi.

Silliness of smart bulbs aside, I would hope the answer is how ipv6 is actually safe for this, not that you should just not use wifi.