Attestation means that whatever code runs on your authenticator has been submitted to the FIDO Alliance (google and buddies) and they signed off on it. After that it can't be changed.

Security keys like the Solokey Hacker (the superior version), is not signed and never will be, cause I can tinker with it in any way I wish. It's truly mine.

Like the article highlights, attestation is part of the standard. The remote service can ask for cryptographic proof that your authenticator, where you stored your passkey, is attested. I have not seen any service doing that so far. IT managers are doing that internally, for their employees for sure. They don't trust bobs homemade solution, only their chosen vendors. From that standpoint it's reasonable.

This gives the service the power to deny you freedom of choice of where to store your passkey. They might say only yubikeys allowed or only microsoft hello or only IOS secure enclave or whatever they wish. It might become a scenario where they don't allow any passkey solution that can run outside their platform. Running LineagOS or Linux? Google and microsoft will say it's "not secure enough".

They will abuse that power to make your decision for you and lock you in. "Password manager not allowed, Use our cloud insted".

Ideally it's not part of the standard and they don't get the option of knowing anything about your authenticator. It's none of their bussines how i choose to store my secrets.

Any sane service will just not ask about the attestation. They don't have to ask. I would not ask my users on my webpage. You can store your passkeys on printed paper for all i care. Although, storing them on a usb/nfc key is pretty damn secure and cool!