The only segment of this pie chart which could reasonably be expected to be hiding missing labor is the not-working-aged-between-16-and-64.
Trouble is, 37.4 million of the 45.7 million in the group are in high school and college. That leaves 8.3 million from this group who could possibly work, many of whom are homemakers,the idle rich, people who have just given up finding a job, volunteers, etc. The government says only 5.2 million of them in May 2018 "want a job now".
The idea that we should think about putting old people and the disabled to work is crazy on its face, while there is something to be said for putting prisoners to work.
Why King isn't rebuking Republicans for wanting to replace Americans with cheap foreign workers ought to be the issue. So-called record job openings go unfilled because they can. The unemployed at 7.2 million plus the 5.2 million represent 12.4 million who would work, if only business paid enough.