> For example, they were talking about the debunked idea of solar panel roadways. If something like that took off in the future, literally every panel would require its own ipv6 address.

Hardly a problem. According to [1], there is 64,285,009 km of roadway in the world. So every meter of road in the world could be addressed with merely a 36 bit integer:

  64285009*1000 = 64,285,009,000 < 2^36 = 68,719,476,736

Remember that with every additional bit, the address space doubles. Exponential growth is insane.

[1] https://en.wikipedia.org/wiki/List_of_countries_by_road_netw...

Which would be fine. Even if you covered the entire earth with 1 square millimeter solar panels, you could give each of them about 66 quadrilion IPv6 addresses.

Why do they need an Internet connection at all? Even if they do need to be connected to the Internet, why can't they connect through NAT?

Because NAT is a hack to cope with IPv4 address space exhaustion. The main benefit of using IPv6 is that it allows you to get rid of NAT.

That might be, but it's a hack that works pretty well on a lot of real-world systems.

If you think that, you've not read the details of any NAT traversal schemes that aren't uPnP.

NAT is a hack that was created due to a lack of addresses.