You're right about the market share; I'm not sure about the motivation. Apple has the capacity to be an attester in this system, since they are an OS and hardware vendor. And while they care some about privacy (in as much as it's a marketing point), they manifestly don't care about user freedom (on iOS, and increasingly on MacOS). I think as long as their code running in a secure enclave is an acceptable attester to this API (which it will be), they don't have any motivation to oppose it.