Thread should not have allowed Internet connectivity by default.
> (..) the ability to commission a finished product into a Matter network in the field mandates certification and membership fees, entailing both one-time, recurring, and per-product costs. This is enforced using a public key infrastructure (PKI) and so-called device attestation certificates.
Matter is designed to necessitate phoning into the internet for certification, which allows all manufacturers to just easily add-in phoning into the internet for telemetry. Furthermore, it does not define exactly which functionality must remain if a device is disconnected from the internet for a long time (assuming the user is trying to avoid sending out telemetry, not even bypassing certification).
It’s the commissioners that check for the certification: your phone or an open source implementation.
And the commissioners can, explicitly by the specification, commission other devices while completely offline.
That's a little disingenuous... All Matter-over-WiFi devices inherently have Internet access unless you're blocking it through router policy, and most Matter-over-Thread border routers commercially available today give Thread devices Internet access by default if they want it (to avoid this you need to self-host your own OTBR instance).