You originally asked what the point is of a self encrypting device. Even assuming this particular device in question has the attack vector you pointed out (I don't know, I haven't checked), it isn't pointless.
Your attack vector assumes the key is in memory. That would be a very odd and weird design that I've never seen... The auth key is usually stored on cryptographically secure hardware (a tpm), you unlock it at boot and it talks to the drive, the OS doesn't know about it or see it.
There's technically an attack vector there, an unproven one that assumes they have a backdoor or an undisclosed flaw at the hardware level... so sure, if you really really don't want to risk it, encrypt your data yourself.
There are a lot of reasons to choose these devices:
- Performance
- Nothing to configure os side, you just write to it and it's encrypted. Power loss means it cannot be decrypted by a malicious actor.
- Disposal. You can often issue a single command that scrambles the auth key and now all the data is unrecoverable without you needing to write every byte.