Not so absolute...
While solid state media cannot retain overwritten information, hard disk drives CAN retain forensically retrievable information. That is why the US Dept of Defense has had long-standing protocols for multiple repeated overwrites of HDDs to completely erase classified information.
To be clear, the reason why it is possible to recover data from a hard disk is because the heads do not precisely track the same position each time that they pass over the disk. This leaves tiny "edges" where the heads wrote data last time but missed this time. This is what the forensics specialists recover.
Multiple overwrites usually solves this problem. So, digital information cannot be retrieved when over-written, but it's not always over-written completely on a hard disk.