Earlier quoted context omitted.
"Engaged" and "disengaged" are terms of art in management which refer to whether someone is totally checked out or not. Someone who spends most of the day on Facebook is "disengaged", even if they're very competent and get more done in an hour than their teammates do in eight. Someone who basically takes their job seriously and puts in a reasonable effort is "engaged", even if they're a total idiot with net negative…
And they're apparently pretty approximate, if being 'disengaged' means that you're only 18% less productive than your 'engaged' coworkers. And then they provide the striking statistic that it will cost an employer up to 2X the annual salary of the 'disengaged' worker to replace them - that's equivalent to ten years of their not working hard, doesn't seem worth it, somehow.
For example, my guess would be that the lost value from a disengaged employee is closer to 100% of their salary than 18%, because of how they drag the rest of the team down by generating bugs, derailing conversations, needing constant help to un-fuck their local env, etc.