You're right that the ECC validation is very robust, but that only validates one small part - that the drive is reading what it has previously written, not that the data was correct when it came in to the drive, correctly handled by the firmware, or even written in the correct place (LBA) on the drive.

There's been times when some features of entire models of drives have been disabled in the Linux kernel because of buggy firmware that silently writes bad data (with correct ECC), so reading it back is successful from both the drive's and the OS's block driver views.

I was hit by this myself with the queued TRIM command firmware bug that affected all Samsung EVO 840 SSDs (Linux kernel commit 9a9324d3969678d44b330e1230ad2c8ae67acf81 if you want to look into the history) - the drive didn't report any errors, but ZFS kept reporting corruption, and kept on fixing it in the background.