I was watching the DVD of Rudolph the Red Nose Reindeer with my daughter last night. I was amused to note that the constraint in the elf toy factory is the paint shop operated by an elf called Herbie - remember the walk in the hills example from The Goal where the slowest boy is also Herbie. How do we know Herbie is the constraint? Because the foreman sees the inventory building up in front of him. Why is Herbie the constraint? Because he is not motivated in his job and his throughput is reduced through lack of proper motivation. Herbie wants to give up being the factory constraint and become a dentist instead. As I've pointed out before, dentists really understand how to be the constraint and how to manage around it.
The rest of the discussion has to do with motivation and this hidden impact on project success. Anderson focuses on software project management, but doesn't motivation affect just about any human project? Whether it is one person writing a book or a massive undertaking with hundreds of people, low motivation leads to much longer-than-expected task times.
Be sure to read the comments, too. There are some other views on motivation as constraint.