[fdeee]

I think you are right in that it is quite impossible to read data that has been overwritten from a sufficiently modern HDD.

But all modern HDDs retire sectors that exhibit a higher correctable error rate and replace them by reserve sectors. Those retired sectors then still contain readable data (maybe with some bit errors) if you know the appropriate factory mode commands. Overwriting the entire disk also won't overwrite retired sectors, except if the disk has an explicit function for it and you trust that function to work properly.

In short, you always need full disk encryption.

[Dalewyn]

Or a low level format.