You are missing the point a bit - I don't think the OP is saying that a bad engineer ("difficult to work with", "don't produce maintainable code", etc) suddenly becomes better when they work by themselves. The point is that one person working alone can sometimes be significantly more productive than if threw more people at the project.

The natural engineering management answer to so many problems is "I need more headcount" - but often that can be the opposite of what the problem needs.