That's how the orange dot is appearing - it's a UI element from the interactive user-interface of macOS, but it's appearing on a video out because they're re-using a monitor signal designed for mirroring or extending you desktop.

That might be pedantic, sure, but the point is that extending is not mirroring.

For the use cases like the one described in the article, that distinction matters. I'm not questioning the usefulness of the orange dot, but rather the people saying the workflow of using a second monitor as output as being invalid and wrong.

You know what I mean and the distinction is meaningless. No one doing a show as big as that is doing it without the hardware necessary to make sure that they can control 100% of the output path.