Actually, they do do something under the Mongoose rules and are, in fact, the co-pilot. Astrogation in Mongoose terms also covers normal space navigation.
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They are not the co-pilot because Pilot skill is not a required part of the job. They ought to be the co-pilot, though. But the character creation system is not necessarily going to make that a thing.
The astrogator has always been the person who does real space navigation, the fact that they have any meaningful role in jump space navigation (instead of the computer doing it) is the innovation of later editions of Traveller. The specific task chain referenced is a Mongoose innovation.
However, there is a very good reason why every sci fi show out there that bothers with a navigator character quickly finds them some thing else to do. Notably, in Star Trek, the navigator was always the noob guy around to be stupid so the real crew could do or say something important. Until Chekov and he was given fire control responsibilities before too long. Star Wars implies that the pilot is doing navigation, but it never happens on screen. And so on.
I have a question about your quote. There is a task for plotting a course using a gravity slingshot. Great. What does that DO? How much faster does that make the trip? How often do you have an orrery for the system the players are in so that your player can even determine if that's useful without you telling them "hey, you can actually use your astrogation skill now for something other than the usual save or die on the way to the adventure?
Classsic Traveller did not have an astrogator role as necessary for basically any player scale ship (Had to be over 200 dtons to need one) because the game offers approximately 0 support for actually using the skill in a fun and interesting way. Mongoose has now made it mandatory that someone on the crew of every starship have it, but has not done anything to make having it as your primary job skill fun. It's just a "please make the saving throw so we don't die on the way to the next cool thing" skill. Worse it managed to be that and also just a modifier to someone else's task check to actually do the thing.
IMHO, the difference between Pilot (starship) and Pilot (Small craft) cascade should be the astrogation training and Astrogation should just go away as a separate skill. Or, if that's too radical, it should be an additional cascade slot on the Pilot skill.