* * *

On some level, you're right. Anything that isn't a zeroed out drive still contains data on it, though. If, say, Apple kept a copy of your drive key or had a backdoored derivation function, recovering your data would be as simple as a call to Cupertino. Failing that, the adversarial Powers That Be can easily generate a disk image to keep around for when AES is broken (or when it's possible to quickly bruteforce it).

This is for people who want to go a step beyond that. The next-best-thing to shredding the disk, you might say.