
I'm guessing you haven't lived with an INTJ... or hell, known one that close.
They are open, sure, compared to an S, but the concept of an INTJ being more flexible than an INTP requires me to suspend everything I know about INTJs or redefine the word 'flexible'... neither of which change that INTPs are vastly more flexible, in all sense of the word, than INTJs.
N does not equal flexible. P/J = flexible.
If you look at the Step II breakdown (paraphrased);
J = Systematic, Scheduled, Methodical
P = Casual, Open-Ended, Spontaneous, Emergent
N = Abstract, Conceptual, Theoretical, Original
S = Concrete, Realistic, Practical, Traditional
T = Logical, Reasonable, Questioning
There is very little question that Ni is considered less flexible than Ne, as a result of NJ and NP respectively. There is nothing in N that causes people to be flexible - Ni being dominant makes them even less flexible... and Ti (TP)certainly holds more "flexibility" that the equivalent TJ.
Meaning that J is the major determinent in how 'flexible' someone is.