Practically speaking yes, the OS (and further down the TPM/enclave) will be the root of attestation. Google here is starting with Google Play Integrity (previously known as SafetyNet), which is an OS-level attestation authority. On Windows, this attestation would probably be done via TPM/Secureboot and Windows integrity APIs.
That's what's scary about it, because it has the potential to make large parts of the web inaccessible unless you have a signed and sealed OS layer and browser to browse it with.
That's what's scary about it, because it has the potential to make large parts of the web inaccessible unless you have a signed and sealed OS layer and browser to browse it with.