Factory restore will not get rid of the data, it will just wipe the file pointers, as a standard delete will do. Any reasonably good data recovery software can easily pick up the actual data - just test w/ a free one (e.g. Recuva) on your current machine.

A truly secure data deletion means you must write 1s and/or 0s into what the computer now believes is ‘empty space’ to clear all possible areas where the data may still reside.

It is theoretically possible that a determined data forensics expert with some heavy-duty physical, magnetism-based tools might be able to access data even after that, but the degree of difficulty is likely already significant after even one pass and 3 is likely well more than needed.
(That 7-10 passes are needed to deny the theoretical expert access may be a myth.)

Of course, that is only if the HDD doesn’t have any physical / software / firmware vulnerability or compromises. I expect shredding the drives is to reduce all possibilities to zero.
(With SSDs now, over-provisioning, ‘hidden’ cells for resiliency or failed sectors pulled out of usage by the SSD controller likely makes physical destruction a necessity, as secure erase is not a certainty.)