It could also mean that they are constantly trying to game people into believing they feel a certain way but they actually don't.
Or it could mean they were taught to do this at a young age, and it's an automatic response.
In any case, it makes one look less human and more likely a manipulator, or at least concerned with the manipulation of people enough to suppress an automatic reaction.
I doubt that many would agree with you that those who react more calmly "look less human". People are generally drawn to those with a calm, positive demeanor, as they are often enjoyable to be around.
Everyone trying to hide their anger all the time leads to fake, sterile interactions, and pent up hostility, in my opinion.
We'd be a lot healthier of a society if we didn't project and expect a perfect image of everyone all the time.
Or, perhaps they managed to reduce their base anger response, and aren't actually hiding anything?