Old "signals" (writes) remain on the magnetic media. This has nothing to do with logical organizations of binary data such as sectors. It's at a lower level. It's all about residual magnetic flux.
Many of the comments at the bottom of the post are informative. I believe the last comment to be correct: given a strong enough reason, forensics can be effective.
The company I mentioned has many levels of recovery services, including the very low level analog stuff.
I hope this helps explain where I was coming from. ;-)
Your link agrees with my position, even the last comment. I briefly touched this in the two posts above yours.