This is my chief worry with Thread. Zigbee is guaranteed to be local only, but if they switch over to Thread, the individual bulbs will be able to call home, even if they expose some of their functionality locally via Matter. With home assistant, one can probably configure their Thread Border Router to not allow internet access, but I have a suspicion a lot of supposedly local thread/matter devices will be designed with the assumption that they have cloud access and won't function fully if firewalled.