Assuming you have an operating system that is running bitwarden and a malicious program that is able to read bitwarden’s memory:
If bitwarden has access to that encryption key stored in a secure enclave, I don’t see how you could prevent that malicious program from doing the same.
You could encrypt the key in that secure enclave, but that meta encryption key would have to be stored in memory, which is accessible to that malicious program.
With malware with OS admin privileges on a system, there is very little (or nothing) that can be done. That’s why operating systems like Qubes OS exist.
And, by the way, if you were paranoid enough to use an OS like Qubes as your daily driver, I don’t think you would be using an online password manager.