Did they get you to run any sort of basic deletion first?
I doubt it- you know the Civil Service, an HDD was only junked when it stopped working
On paper that process described is adding a great deal of "surface" for the disk or platters to go missing or be stolen.
You can't get data off a platter that has no magnetic material on it... I don't think they cared about the waste grit, on the principle that if someone wanted to extract all the magnetic dust and reconstruct the data (cf
your shredder comparison) they were welcome to try
Talking of shredders... shredded classified documents were collected and incinerated on site, again with witnesses signing it off. Ditto for floppies, which were still in use at the time. And you stood more chance of winning the lottery three weeks on the trot than getting a writeable CD drive on your computer.
And we haven't even mentioned Tempest protection!
Paranoid they may been, thorough they certainly were.