The key is found in the exploit label and in the combat master's maneuver section. The first ball of Martial Adept says (PHB, p.168, it's me who points out):
You learn two maneuvers of your choice from those available for the Master Battle archetype in the fighter class. […]
The second paragraph of the section on the combat master's maneuvers that you quoted says (PHB, p.73, it's me who points out):
You learn two more maneuvers of your choice at levels 7, 10 and 15. Each time you learn new maneuvers, you can also replace one maneuver that you know by another.
Since Martial Adept allows you learn For additional maneuvers, the Combat Master function allows you to swap one of your previously selected maneuvers with another. The feat also grants you two extra maneuvers and one extra die of superiority. No additional combat level required.