I don't agree with the statement that a glut in one area must equal a surplus elsewhere. You've added at interpretation to my statement.
If a person produces hot pink sweaters and finds nobody wants them, that doesn't mean there is a shortage of blue sweaters, or even sweaters in general.
Additionally, it might mean that people might want hot pink sweaters, but not at the price being asked.
Taking this back to labor markets, surplus labor (unemployment) is indeed oversupply, that much we are agreed. But you must break this down further - it is an oversupply of specific types of labor at specific rates. You could clear that labor oversupply by switching the supply by changing the type of labor being offered, and also by changing the price. For example I'd happily pay someone to do work for me at lower rates than currently offered, but that market doesn't usually clear. Changing the price doesn't happen often for regulatory reasons or stickiness, but changing the nature of labor supply often does. It's just that the timescales involved involve a lot of problems.
The solution to that is to identify reason why mal investment occurs and to avoid anything that contributes to it. I don't think it can be totally avoided due to human nature - hubris, mistakes and misfortune - but you can certainly avoid some of the obvious ones, like rigging credit markets and trying to centrally plan economies.