First of all, if someone has unauthorized physical access to your device, you're pretty hosed. Especially if they happen to have a current logged in session. Forget passwords, they have cookies and mail.app and bookmarks. Second, if you must lend your computer to an untrusted person, use the Guest session. I just tried this and confirmed that there is no immediately obvious way for a person logged in to a guest session to access my keychain.

Knowing this, I will rewrite the opening line to the article.

Original line: "Here’s a reason why you shouldn’t let anyone use your computer."

My revised line: "Here's a reason why you shouldn't let [untrusted persons] use [a non-guest session on] your computer."