The assertion is that a Mac user can run their own Mac as an admin, with no higher risk than running a standard account, which is entirely true. Your argument relates to network administration, which is not part of the assertion or this discussion. Everything being discussed here relates to average Mac users running their own Macs, not to network administrators. It helps if you stay on topic.
This has nothing to do with network administration, as this is relative to a single machine, not a network nor networked machines together. But let's take your single user scenario into consideration.
You assume the bold. as a Sysadmin, you can not assume this, because you do not know the level of knowledge any other user has. So let's say that as an admin account, you let someone else use your Mac, after logging in successfully. In the time that they are using your Mac, they could copy out important information (password keychain, quick copy of your password file, etc.), quickly edit your passwd file and shadow file to remove any password string, etc., change your password so you are effectively locked out of your Mac, so on and so on. You don't know the limit of their knowledge, so you shouldn't assume that you do.
You want to talk about systems and network administration being irrelevant to the discussion at hand? This has nothing to do with those, but more to do with systems administration and security, which has everything to do with any type of administration, whether it be for Unix, Linux, Mac, Windows, VMS, or otherwise.
You really are not understanding the ramifications of what power an admin account has and how it can be exploited in the wrong hands due to negligence of the user.
BL.