Interesting theory. My experience is not that humbleness is enforced by the boss, in order to keep his people in line. With the exception of really passive-aggressive bosses perhaps.
Humility is taught not by bosses, but by colleagues, friends and family. By pretty much all around us: "nobody likes a showoff" is not something your boss is going to say, typically.
The crucial observation is that in order to be humble you have to *undersell* your achievements. If you are neither humble nor a braggart you present your achievements accurately to the rest of the world, and that is frowned upon. So why is it that we are not supposed to speak frankly about our achievements?
I think the issue of humility is completely explained by the concept of Status Illegibility (as defined in http://www.ribbonfarm.com/2010/10/14/the-gervais-principle-iv-wonderful-human-beings/).
If I everybody in a group of friends (or family) speaks clearly about their accomplishments, then it becomes apparent who has accomplished a lot and who hasn't. This establishes a clear hierarchy in the group, a ranking with the winners at the top and the losers at the bottom. Only the few at the top are happy with such a ranking, the others will feel resentment. So the group will become unstable and break apart (see linked article). This also explains why people typically find new friends when they become really successful (so successful that no amount of humility can conceal it). People don't like to be confronted with the fact that their friends (or colleagues or family members) may be better than they are, so a social stigma forms against showcasing your achievements frankly and accurately.