I’m a Sonic customer, and have a fully working IPv6 connection from them.

How?

https://help.sonic.com/hc/en-us/articles/115000419168-Fiber-...

Gigabit fiber from them explicitly says it isn't supported.

Whoa. I have no idea. I absolutely, 100% certainly, do though. I have Sonic gigabit fiber, and can use IPv6 to connect to remote hosts (and can connect to my LAN from remote hosts on specific ports I have open).

If your IPv6 prefix begins with 2602:240, then your router is using the Sonic 6RD tunnel.

It doesn't. I'm in the 2600:1700 network.