Doesn't passkeys give the service a signature to prove what type of hardware device you're using? e.g. it provides a way for the server to check whether you are using a software implementation? It's not really open if it essentially has type of DRM built in.

You're thinking of hardware-backed attestation, which provides a hardware root of trust. I believe passkeys are just challenge-response (using public key cryptography). You could probably add some sort of root of trust (for example, have the public key signed by the HSM that generated it) but that would be entirely additional to the passkey itself.

Passkeys do have the option of attestation, but the way Apple at least do them means Apple users won't have attestation, so most services won't require attestation.