The article shows writing out a modified partition table, getting the message:

WARNING: Re-reading the partition table failed with error 16: Device or resource busy.
The kernel still uses the old table.
The new table will be used at the next reboot.

and then, without rebooting,  trying to write to the new partitions (running "mdadm -add ...").

Doing that is extremely dangerous to any data on that disk---and even if there is no data, doing that means  mdadm might not be initializing something (the kernel's old view of partition N) other than what you meant (your new partition N).