That was exactly the example given in the article, but somehow this isn't what I expected would happen if I click a link in say, my email client or chat program.

I imagined it more like: User clicks link in email program. Email program tells OS: "Here, open https://..." -- OS checks URL scheme registry and selects Firefox, OS brings Firefox to the front and throws the URL at it and says "Open this."

I guess perhaps my naïve way could falls down if the OS accepts URLs from apps that aren't in the foreground, so a random background process could activate any app it wants to steal focus.