The absence of any given technique in the kata of a system but which is within the curriculum of the system does not, by default, invalidate the thesis that kata is a library of techniques. However, the presence of all of the system's techniques within the system's body of kata would, as I stated, be a powerful argument in the "kata as a library of techniques" thesis.
This, again, goes to my position that kata is many different things to different people and different systems. I have several friends who practice Silat. They tell me that there are countless discrete and independent "village styles" of Silat. Many of these, apparently, are represented, in entirety, by a single form/kata which the village practices. From what I can tell, to them, the form IS the system, inclusive.
And yet, in this thread alone, we've had 4-6 different descriptions of what kata "really is." My conclusion is that "they're all right." Kata can be any number of different things from teaching core principles, through choreography of a fight, to being a library of techniques. Kata is represented by a set [a..z] in which nearly every potential description is represented.
There is no wrong answer. Kata is a Rorschach blot.
Peace favor your sword,
Kirk