It’s not security theatre in the rare case that the stated goals is to protect against physical attacks, or for hardware disposal.
If the goal is to put “military grade encryption” sticker in the sales deck, or to pass some certification, or to create a vague cloud of plausibility that you’re taking security seriously in some other way, then absolutely it’s security theatre. It deserves to be called out.
It does protect against physically taking of and improper disposal of HDs when done correctly. Which can be quite a significant vector.