Yes, this exactly. Tethering does give only IPv4 addresses and sets up routes and NAT for IPv4. Somewhere in the future IPv6 support can be implemented. Or if someone wants to try out feel free to wander into ConnMan code…
If you want you can try as much as you are capable of. If you listen for the D-Bus property changed signal “Tethering” as a boolean value from connman/connman/doc/technology-api.txt at master · sailfishos/connman · GitHub and then in a script make the necessary changes to enable all the necessary bits for IPv6 in kernel and iptables, and then revert them when the property has changed back to “false”, we surely would like to see the process you managed to do. It would be then easier to include it into the codebase of ConnMan as well. There are other more urgent things to do and this is far from the top of the priority list.