The "disable" section of the "Restore Gatekeeper’s ‘Anywhere’ Option" section of your linked doc displays an incorrect command. (Same -- error at the bottom in the "enable" section. Proofreader needed...badly! Inquire within.)
Enter the following command at the Terminal prompt (note: before “master-enable†there should be two short dashes with no space between):
sudo spctl –master-disable
(Emphasis added)
sudo spctl --master-disable