This is an attribute that is entirely concerned with screen reader accessibility. Specifically, it allows certain scoped exceptions to the screen reader behavior of only enabling interaction with a single element at a time.

The thing about ARIA is that it does not affect browser behavior or functionality -- only semantics and accessibility API mappings. All keyboard events will still fire on the true focused element, and there are no global DOM methods to query the currently relevant active descendant in the manner of document.activeElement. The only context in which the active descendant behaves like a second focus is when it comes to a screen reader's virtual cursor.

Even for screen readers, aria-activedescendant is not entirely the same as a second focus.