I'm guessing that the GP is talking about the fact that if there were two persons in a household using the Internet at the same time, with IPv4 they would connect from the same IP address (though of course with different port numbers), but with IPv6 they would likely connect from distinct IP addresses, and usually only sharing a /64 prefix.

You are correct that this isn't a big issue. SLAAC addresses are generally changed fairly frequently by the OS.

IPv6 privacy extensions are only switched around once a day or so in most default configurations.

You can change that, of course, and switch addresses every minute if you want to, but I do find the default a little high.

Once a day is still way more frequent than most routers switch IPv4 addresses, so I'm not sure that the net result is any worse.

The difference is that the average household shares multiple devices behind that single IP address, whereas IPv6 addresses are unique to the device you're using.

Identification to the level of IPv4 can still be done with IPv6 by using the /64 where you would previously take the /32, but with IPv6 you also get identifiers from within the network as well.

With how much IPv6 space is available, I'm not sure why SLAAC-based networks don't just assign different IP addresses to different use cases. I can see this becoming a problem on large company networks, but in home networks you could generate a random IPv6 address every hour for every website you visit and still never run out of address space.

Operating systems aren't exactly geared up for per-application outgoing IP addresses, and perhaps handling tens of thousands of IP addresses will bog down the kernel somehow, but in terms of privacy protection we could be doing a lot more than what IPv6 Privacy Extensions are doing right now.