Actually, unlike at the Hauptbahnhof, it is not Tief, but Terminal, as opposed to Besucherpark, the other S-Bahn stop at the airport.

A good reason for changing to Salzburg at the Ostbahnhof, as opposed to at the Hauptbahnhof, is that the Tief-part of Hauptbanhof (where the S-Bahn stops) is some distance from the mainline tracks whence the Salzburg trains leave, and the entire set-up at Hbf is sprawling and rather confusing for first-time visitors.