They're only emotionless compared to the significantly more volatile 1w2s. My roommate all of last year was an INTJ 1w9 and she was MUCH better at holding in anger than I was (as an ESTJ 1w2). It helps that 1w9s are generally introverts, while 1w2s can go either way.
And from my experience with my roommate last year, I can say that when we each let out our anger, it took very different forms -- though I'm sure the emotion behind it was equally strong with each of us. With me, it would be an extremely harsh self-imposed guilt trip, often out loud, and sometimes with tears: "There were all these things I could have done to make everything work out, but I had to be a stupid, irrational idiot and choose the worst possible option. Someone needs to lock me in a room and keep me from making any important decision ever again." The repressed anger was tinted with 2 guilt, and was aimed at myself.
Whereas, with my roommate, her anger outbursts would be cold, almost lecture-like rants at the system as a whole: "This person is against me. Everyone is against me. The entire system is designed to make this happen every single time. It's all hopeless. Fuck the world, fuck the system, I'm moving to the wilderness and becoming a hermit." The repressed anger was tinted with the 9 urge to be as neutral and detached as possible.
I don't know how much this reflects other 1w9s.