If labor is being used efficiently, there is no excess unemployment or over employment.
This phrase from Khan Academy struck me as quite strange. Why on Earth would over employment mean that labor is NOT used efficiently? Labor is a resource. And if we can use more of a resource without increasing its size, then it would seem that we are using it more efficiently, we are using bigger share of it.
For an example, land is recourse. If we will drain swamps we will be able to use more of land for commerical purposes, meaning that we will use land more efficiently. I believe that similiar reasoning can be applied to labor.
P.S. What I understood from comments: Overemployment is inefficient because it happens due to sticky wages/prices in short run equalibrium. Sticky prices/wages create inefficiency in the market, thus it should be nothing suprising that overemployment caused by sticky wages/prices leads to inefficient use of labor.