Hi It would indeed normally take many hours to erase the entire disc into unrecoverable unreadability by overwriting every sector. (which is why the military prefers a small demolition charge
)
However if you (or apple) had decided
- (File Vault rings a bell as the setting but third party tools were also available before that)
to encrypt the drive then the contents are already gibberish to anyone without the encryption/decryption key. The secure erase task is then vastly simplified to overwriting the keys in the crypto system (not the whole drive) numerous times.
I cannot say that your iMac has this feature or that it is on etc. but it may be somewhere in the appropriate support sites or product information.
The same thing causes surprise when you ask a newer iPhone (all have encrypted storage from day 1 ) to securely erase 128GB of storage before repair or resale and it immediately says "done it"
Why not try to set it up as a new machine yourself and see if any trace of 'old you' remains.