Passkeys protect against credential theft, not session hijacking, two different parts of the stack. Passkey's only concern is initial authentication, it was never meant to provide any sort of protection against session theft. RFC 9449 Proof of Possession is how you prevent session hijacking, or session binding with a client-side TLS certificate.

Device-bound passkeys take care of non-repudiation. With synced keys (e.g., 1password), an account compromise of your vault hands the attacker all your credentials, the private keys are in the vault.

Device-bound keeps the private key sealed in the TPM (or secure enclave), the key cannot be exported, so it cannot be extracted remotely. Even malware on the machine, can hijack your session, but it cannot exfiltrate your private key, TPM won't release it to the service without user verification via biometrics, yubikey, or a PIN. There's also an attestation chain that breaks with synced passkeys. The attacker has no way to get your private key, so the only way to compromise the account is, yes, session hijacking, or physical access to the device with the user present to pass the biometrics check.