I agree, some people are shy, unsure, etc, and will get defensive when presented with an angry person barking orders. It's not the military. They may find these people won't even talk about their work because they don't want to offend the barker.
That said, it really depends on the culture of the employees on which motivational techniques to use.
For me it didn't feel like something from the army as much as a car salesman trying much too hard.
Also, as a college student, I would be really suspicious of somebody telling me not to go to class--I'm perfectly happy to do that of my own violation, but anybody suggesting it immediately comes out as against my best interests.
It's more of an analogy like this: You're a group leader for a group assignment due Tuesday morning. You think it's great, they're the top 1% in the class. They promise to do their sections of the topic by Monday meeting so you can work together to compile it. Monday comes around, they said they tried and didn't finish their sections. Now you're stuck all night fixing their sections and compiling it together.
That's simplified analogy, and the author would angrily call them "jokers".
Except for the bit where you're doing other schoolwork instead, I presume? Besides, I wasn't talking as much about the content of the letter as the tone--that's what I found off-putting.
That said, it really depends on the culture of the employees on which motivational techniques to use.