OK I see the differentiation you are making TWAS. I guess the query is that when there is a vacant spot caused by an injury to one of the incumbents, everyone moves up a place, so is the spot at the bottom of the ladder then given to an emerging player or a currently ready player.
So rewinding back to 2014, Fardy was the incumbent 6, from memory Jones and McCalman filled the void initially and then McMahon was given a shot in the Barbarians game. At the time McMahon was relatively unheard of, so they could have considered other lowly capped Wallabies for the spot such as Jake Schatz (or did he go anyway?). Anyway, I think in an instance like that they elected for some new blood, whether it was because he was considered the superior player for that tour we just don't know.
Looking at the tour squad, you'd think that the injury to Kerevi created a spot for either Godwin or Koroibete, one of them probably wouldn't be going without the injury, so in that sense you are correct. But they are both uncapped.