This is not entirely true.
We're not talking about rocket science.
Please remember, a hard drive is magnetic media, north:south, or yes:no. It's easy to re-arrange these two variables sufficiently to remove the previously arranged state of the disk. This new arrangement renders the old state oblivious to the new state.
Low level recovery is possible.
There are some minor caveats here though. Magnetic media has a current state but there are often signs of a previous state. But the present rate of recovery is abysmal, roughly 25% recovery.
Government agencies and some disk recovery companies fill in the blanks using tempest data, guess work and plan old templates.
Also keep in mind, that restored data is not admissable in federal court. The restorative rates are too low to be considered valid.