>> If an option is highlighted, never change it, even if new data is loaded

>I don't get this one. If I have the second item selected, and search for a completely new term, should the item remain wedged in the new results at the same position?

I think the author means "never change it without user input". The problem is that Autocomplete is often slow, so developers load suggestions asynchronously, and may update suggestions shown to the user after better suggestions have been loaded.

If a suggestion was already highlighted at that point, the system should not change it, to avoid the case where the user tries to commit a suggestion, but the system changes it just before the user hits the key to commit the suggestion.