There is a rich history of IoT devices using IPv6 to communicate among themselves without relying on the cloud. I think Nest started this trend. One Nest device sends a specific RA to make itself the router of all other Nest devices. All other devices can configure themselves thanks to SLAAC. The benefit of v6 is that there are so many addresses out there that the Nest device can just pick an arbitrary ULA and there won’t be collisions.

Don’t know about Matter though. If it requires the user to turn on IPv6 then it’s a user experience downgrade. It should just use IPv6 internally as an implementation detail.