It means malware can’t exfiltrate the SSH key from your machine and keep using it. But yes they can potentially use it while still on your machine depending on if presence confirmation or re-inputting a credential is required. But that still closes a big gap.

On a Mac secretive can also pop a notification making it more likely to passively observe such usage (not fool proof though) and the key can’t (easily, maybe with some complex exploit) be used from an app not signed by the original developer. It can also require re input of your password. That specific security probably isn’t possible on Linux though.