You could just read the disk again and check for nonzero blocks. If you don't trust the disk to read or write to itself either, then you might as well just toss it.
It wouldn't be that technically complex to have modified firmware that looks for the secure erase command (not something that's ever invoked except purposefully by the user), and returns all random for read sectors without ever actually overwriting the underlying data.
But yeah, you're right. If you ever have reason to invoke the secure erase command, you're probably in a position where the next step is throwing the drive in a shredder.
