Preparing my Mac for sale: deleting/clearing disk in Disk Utility
That is the recovery image that you reinstall macOS from in recovery mode (assuming you don't have anything else plugged into your Mac).

You can safely ignore it as it is just an installer.

You have wiped the drive. Boot into recovery mode and reinstall macOS. That will have the added benefit of overwriting the disk with new data making recovering your personal data all that more difficult to recover by the next owner.