They're also injecting a CA bundle so, presumably, they're in including their own root of trust so they can sign their own certificate. I'm on mobile and can't easily look at what they're including.

Edit: Guess I've got openssl in my termux environment. They're injecting a fake Nest root CA key. Makes sense.

I'm shocked it was this easy to subvert the root of trust on these devices. I would expect a newer device would have the trust root pinned in hardware (TPM, etc) and firmware updates would be have been authenticated.