How the recommendation works
The advice follows the rules most storage administrators use. With two drives, a mirror is the only option with protection. With large drives (8 TB and more), rebuilds take a day or longer, so single-parity RAID 5 is avoided in favour of RAID 6 or RAID 10. Speed points to RAID 10, which writes faster and rebuilds by copying one drive. Space points to parity RAID, which loses a fixed one or two drives however many you add.
It is a starting point, not a verdict. Check the numbers for your exact drives in the capacity calculator, and remember that no RAID level replaces a backup.