It's a basic principle of security. Each account represents one person so that you have a full audit of who did what by watching the activity of a given user account. If everything is run as "devops" user for example, you have no idea who actually performed a given task. Was it Bill, or was it an automated job? PCI-DSS requirements also affect your model for user accounts (hint: shared users are often not compliant).
From the perspective of a sysadmin, this article has a lot of issues and it's inadvisable to follow its recommendations. Who doesn't use a hardware firewall? Who exposes ssh to the internet (requiring fail2ban) when a VPN server is much more secure and easier to use? Setting up an LDAP server is really easy and costs nothing. There's no excuse for shared accounts.