First, why would you need to do that? Scrubbing only needs to be done yearly for most workload situations, when an error is discovered or when resilvering.
This book is intended for anyone responsible for setting up and administering ZFS file systems. Topics are described for both SPARC and x86 based systems, where appropriate.
docs.oracle.com
Doing so monthly is not needed and is a COMPLETE waste of time and resources.
Once a month is the default for example in Debian which uses ZFS-on-Linux (OpenZFS). Nobody uses Oracle's ZFS apart from Oracle due to licensing. I'm not saying that their documentation is wrong, but rather that it is aimed at a different class of hardware with different assumptions.
Scrubbing often with very high capacity drives is advised, especially for this drive that has worse error rate spec than even WD Green, not to mention the enterprise drives which are two
orders of magnitude better rated.
Second, that number is a load of nonsense. The "scrubbing" functions require only as much space as there is data on the drive. Nothing more, nothing less. Why? Because scrubbing is just reverifying data on the array and correcting errors if found(rare).
That's why I wrote "If you fill it up", obviously meaning "use the whole 20TB".
Third, you assume that everyone using these drives in a NAS with ZFS implementation will be using RAID1.
It's an easy to understand example.
Most do not, even in NAS racks with only 2 drive bays
This drive is not meant for those according to WD, it's a "Pro" HDD for bigger NASes with up to 24 drives, which I wrote as well. I've used RAID10 in bigger NASes due to performance characteristics, so it's not as clear cut.
Most use some form of striped array with parity such as RAIDZ-1, RAIDZ-2 or RAIDZ-3, depending on the options offered by the NAS device.
That changes nothing, since those are scrubbed as well. And so is btrfs used, for example, by Synology in their NAS appliances.
Parity raids also can have higher read amplification than RAID1 which use up more of the workload, while RAID1 can read different data from different mirrors at the same time. I wrote "can have" because RAIDZ behaves a bit different than classical parity raids.
It's getting mighty tempting...
I'm yet to see you provide a compelling argument, so go ahead