As I see it, perhaps the core concept of poise is that you don’t allow your emotions to control you. Of course you feel them, and of course often you’ll choose to express them because you want others to know what you’re feeling… but the important thing is that it is a choice, a decision. Head rules over heart. Emotions are motives, yes, but you don’t let them push you into immediate action: instead you make considered actions with the aim of achieving the things you desire or ridding yourself of what you despise, etc.
When your friends and family are murdered by barbarians… well, maybe right then you have some important things you have to do immediately (such as writing the report and sending it off, or working to patch up a defensive wall), and poise means doing those things, postponing the grief and weeping until later when there’s time. But then you should grieve because you know quite rationally that it would be bad for you to try to avoid it indefinitely.
I sort of see what you mean about Urizen and the League… but the difference is that in Urizen people usually just choose how much of their underlying true emotion to show (typically expressing it just briefly if it’s strong) whereas in the League, to don a mask is is to present a different self, sometimes thoroughly different. (Not necessarily a fake one, just a different one.)
Urizeni do engage in joking.
Of course people in other nations can enjoy making jokes mocking the parody notion of Urizeni as humourless killjoys who do not do fun… but it’s not really right.