No the directions are not for an 8TB drive. They should work on almost any sized drive. The line: mkpart primary 8591MB -1MB is (IIRC) what instructs mkpart program to use the full size of the drive for the user data partition.
Those commands are setting up the various partitions that the My Cloud OS uses/needs. The mkpart primary 8591MB -1MB line is telling the partition program to use all the space on the drive (regardless of size) above 8591MB partition start point for that partition. All the other partitions are created below the 8591MB partition start point.
Typically you would use those same commands regardless of drive size.
Maybe it would be easier to say that the 8591MB -1MB command says to make the partition
starting at 8591MB thru the end of the disk whether the disk is 6TB or 8TB or 10TB.