That statement by the author and many here on FR is one of the most ridiculous statements ever heard.
Having served with both, I never saw any difference.
IMHO it doesn't make a difference if the force is made up of volunteers or not.
WHAT does matter is the leadership in the units.
In the early 70's I was assigned to units that were a mix of both and they were squared away because they had leaders who took care of them.
In the 80's I was assigned to units that the soldiers were there because they voluntarily signed on the dotted line. The leadership was sorry and as a result these units were mucked up like a dripping soup sandwich.