I think this distinction is very important, for the following reason: Once you've acknowledged that firewalls are technically optional, but used in 99% of cases, there's no reason to say why home routers wouldn't come out of the box with IPv6 firewalls in 99% of cases too.

And this is in fact what we see in the real world with IPv6 deployments. Roughly 50% of my country has IPv6, and every single provider provisions it with sensible default firewall rules.